1=======================
2The Userspace I/O HOWTO
3=======================
4
5:Author: Hans-Jürgen Koch Linux developer, Linutronix
6:Date:   2006-12-11
7
8About this document
9===================
10
11Translations
12------------
13
14If you know of any translations for this document, or you are interested
15in translating it, please email me hjk@hansjkoch.de.
16
17Preface
18-------
19
20For many types of devices, creating a Linux kernel driver is overkill.
21All that is really needed is some way to handle an interrupt and provide
22access to the memory space of the device. The logic of controlling the
23device does not necessarily have to be within the kernel, as the device
24does not need to take advantage of any of other resources that the
25kernel provides. One such common class of devices that are like this are
26for industrial I/O cards.
27
28To address this situation, the userspace I/O system (UIO) was designed.
29For typical industrial I/O cards, only a very small kernel module is
30needed. The main part of the driver will run in user space. This
31simplifies development and reduces the risk of serious bugs within a
32kernel module.
33
34Please note that UIO is not an universal driver interface. Devices that
35are already handled well by other kernel subsystems (like networking or
36serial or USB) are no candidates for an UIO driver. Hardware that is
37ideally suited for an UIO driver fulfills all of the following:
38
39-  The device has memory that can be mapped. The device can be
40   controlled completely by writing to this memory.
41
42-  The device usually generates interrupts.
43
44-  The device does not fit into one of the standard kernel subsystems.
45
46Acknowledgments
47---------------
48
49I'd like to thank Thomas Gleixner and Benedikt Spranger of Linutronix,
50who have not only written most of the UIO code, but also helped greatly
51writing this HOWTO by giving me all kinds of background information.
52
53Feedback
54--------
55
56Find something wrong with this document? (Or perhaps something right?) I
57would love to hear from you. Please email me at hjk@hansjkoch.de.
58
59About UIO
60=========
61
62If you use UIO for your card's driver, here's what you get:
63
64-  only one small kernel module to write and maintain.
65
66-  develop the main part of your driver in user space, with all the
67   tools and libraries you're used to.
68
69-  bugs in your driver won't crash the kernel.
70
71-  updates of your driver can take place without recompiling the kernel.
72
73How UIO works
74-------------
75
76Each UIO device is accessed through a device file and several sysfs
77attribute files. The device file will be called ``/dev/uio0`` for the
78first device, and ``/dev/uio1``, ``/dev/uio2`` and so on for subsequent
79devices.
80
81``/dev/uioX`` is used to access the address space of the card. Just use
82:c:func:`mmap()` to access registers or RAM locations of your card.
83
84Interrupts are handled by reading from ``/dev/uioX``. A blocking
85:c:func:`read()` from ``/dev/uioX`` will return as soon as an
86interrupt occurs. You can also use :c:func:`select()` on
87``/dev/uioX`` to wait for an interrupt. The integer value read from
88``/dev/uioX`` represents the total interrupt count. You can use this
89number to figure out if you missed some interrupts.
90
91For some hardware that has more than one interrupt source internally,
92but not separate IRQ mask and status registers, there might be
93situations where userspace cannot determine what the interrupt source
94was if the kernel handler disables them by writing to the chip's IRQ
95register. In such a case, the kernel has to disable the IRQ completely
96to leave the chip's register untouched. Now the userspace part can
97determine the cause of the interrupt, but it cannot re-enable
98interrupts. Another cornercase is chips where re-enabling interrupts is
99a read-modify-write operation to a combined IRQ status/acknowledge
100register. This would be racy if a new interrupt occurred simultaneously.
101
102To address these problems, UIO also implements a write() function. It is
103normally not used and can be ignored for hardware that has only a single
104interrupt source or has separate IRQ mask and status registers. If you
105need it, however, a write to ``/dev/uioX`` will call the
106:c:func:`irqcontrol()` function implemented by the driver. You have
107to write a 32-bit value that is usually either 0 or 1 to disable or
108enable interrupts. If a driver does not implement
109:c:func:`irqcontrol()`, :c:func:`write()` will return with
110``-ENOSYS``.
111
112To handle interrupts properly, your custom kernel module can provide its
113own interrupt handler. It will automatically be called by the built-in
114handler.
115
116For cards that don't generate interrupts but need to be polled, there is
117the possibility to set up a timer that triggers the interrupt handler at
118configurable time intervals. This interrupt simulation is done by
119calling :c:func:`uio_event_notify()` from the timer's event
120handler.
121
122Each driver provides attributes that are used to read or write
123variables. These attributes are accessible through sysfs files. A custom
124kernel driver module can add its own attributes to the device owned by
125the uio driver, but not added to the UIO device itself at this time.
126This might change in the future if it would be found to be useful.
127
128The following standard attributes are provided by the UIO framework:
129
130-  ``name``: The name of your device. It is recommended to use the name
131   of your kernel module for this.
132
133-  ``version``: A version string defined by your driver. This allows the
134   user space part of your driver to deal with different versions of the
135   kernel module.
136
137-  ``event``: The total number of interrupts handled by the driver since
138   the last time the device node was read.
139
140These attributes appear under the ``/sys/class/uio/uioX`` directory.
141Please note that this directory might be a symlink, and not a real
142directory. Any userspace code that accesses it must be able to handle
143this.
144
145Each UIO device can make one or more memory regions available for memory
146mapping. This is necessary because some industrial I/O cards require
147access to more than one PCI memory region in a driver.
148
149Each mapping has its own directory in sysfs, the first mapping appears
150as ``/sys/class/uio/uioX/maps/map0/``. Subsequent mappings create
151directories ``map1/``, ``map2/``, and so on. These directories will only
152appear if the size of the mapping is not 0.
153
154Each ``mapX/`` directory contains four read-only files that show
155attributes of the memory:
156
157-  ``name``: A string identifier for this mapping. This is optional, the
158   string can be empty. Drivers can set this to make it easier for
159   userspace to find the correct mapping.
160
161-  ``addr``: The address of memory that can be mapped.
162
163-  ``size``: The size, in bytes, of the memory pointed to by addr.
164
165-  ``offset``: The offset, in bytes, that has to be added to the pointer
166   returned by :c:func:`mmap()` to get to the actual device memory.
167   This is important if the device's memory is not page aligned.
168   Remember that pointers returned by :c:func:`mmap()` are always
169   page aligned, so it is good style to always add this offset.
170
171From userspace, the different mappings are distinguished by adjusting
172the ``offset`` parameter of the :c:func:`mmap()` call. To map the
173memory of mapping N, you have to use N times the page size as your
174offset::
175
176    offset = N * getpagesize();
177
178Sometimes there is hardware with memory-like regions that can not be
179mapped with the technique described here, but there are still ways to
180access them from userspace. The most common example are x86 ioports. On
181x86 systems, userspace can access these ioports using
182:c:func:`ioperm()`, :c:func:`iopl()`, :c:func:`inb()`,
183:c:func:`outb()`, and similar functions.
184
185Since these ioport regions can not be mapped, they will not appear under
186``/sys/class/uio/uioX/maps/`` like the normal memory described above.
187Without information about the port regions a hardware has to offer, it
188becomes difficult for the userspace part of the driver to find out which
189ports belong to which UIO device.
190
191To address this situation, the new directory
192``/sys/class/uio/uioX/portio/`` was added. It only exists if the driver
193wants to pass information about one or more port regions to userspace.
194If that is the case, subdirectories named ``port0``, ``port1``, and so
195on, will appear underneath ``/sys/class/uio/uioX/portio/``.
196
197Each ``portX/`` directory contains four read-only files that show name,
198start, size, and type of the port region:
199
200-  ``name``: A string identifier for this port region. The string is
201   optional and can be empty. Drivers can set it to make it easier for
202   userspace to find a certain port region.
203
204-  ``start``: The first port of this region.
205
206-  ``size``: The number of ports in this region.
207
208-  ``porttype``: A string describing the type of port.
209
210Writing your own kernel module
211==============================
212
213Please have a look at ``uio_cif.c`` as an example. The following
214paragraphs explain the different sections of this file.
215
216struct uio_info
217---------------
218
219This structure tells the framework the details of your driver, Some of
220the members are required, others are optional.
221
222-  ``const char *name``: Required. The name of your driver as it will
223   appear in sysfs. I recommend using the name of your module for this.
224
225-  ``const char *version``: Required. This string appears in
226   ``/sys/class/uio/uioX/version``.
227
228-  ``struct uio_mem mem[ MAX_UIO_MAPS ]``: Required if you have memory
229   that can be mapped with :c:func:`mmap()`. For each mapping you
230   need to fill one of the ``uio_mem`` structures. See the description
231   below for details.
232
233-  ``struct uio_port port[ MAX_UIO_PORTS_REGIONS ]``: Required if you
234   want to pass information about ioports to userspace. For each port
235   region you need to fill one of the ``uio_port`` structures. See the
236   description below for details.
237
238-  ``long irq``: Required. If your hardware generates an interrupt, it's
239   your modules task to determine the irq number during initialization.
240   If you don't have a hardware generated interrupt but want to trigger
241   the interrupt handler in some other way, set ``irq`` to
242   ``UIO_IRQ_CUSTOM``. If you had no interrupt at all, you could set
243   ``irq`` to ``UIO_IRQ_NONE``, though this rarely makes sense.
244
245-  ``unsigned long irq_flags``: Required if you've set ``irq`` to a
246   hardware interrupt number. The flags given here will be used in the
247   call to :c:func:`request_irq()`.
248
249-  ``int (*mmap)(struct uio_info *info, struct vm_area_struct *vma)``:
250   Optional. If you need a special :c:func:`mmap()`
251   function, you can set it here. If this pointer is not NULL, your
252   :c:func:`mmap()` will be called instead of the built-in one.
253
254-  ``int (*open)(struct uio_info *info, struct inode *inode)``:
255   Optional. You might want to have your own :c:func:`open()`,
256   e.g. to enable interrupts only when your device is actually used.
257
258-  ``int (*release)(struct uio_info *info, struct inode *inode)``:
259   Optional. If you define your own :c:func:`open()`, you will
260   probably also want a custom :c:func:`release()` function.
261
262-  ``int (*irqcontrol)(struct uio_info *info, s32 irq_on)``:
263   Optional. If you need to be able to enable or disable interrupts
264   from userspace by writing to ``/dev/uioX``, you can implement this
265   function. The parameter ``irq_on`` will be 0 to disable interrupts
266   and 1 to enable them.
267
268Usually, your device will have one or more memory regions that can be
269mapped to user space. For each region, you have to set up a
270``struct uio_mem`` in the ``mem[]`` array. Here's a description of the
271fields of ``struct uio_mem``:
272
273-  ``const char *name``: Optional. Set this to help identify the memory
274   region, it will show up in the corresponding sysfs node.
275
276-  ``int memtype``: Required if the mapping is used. Set this to
277   ``UIO_MEM_PHYS`` if you you have physical memory on your card to be
278   mapped. Use ``UIO_MEM_LOGICAL`` for logical memory (e.g. allocated
279   with :c:func:`kmalloc()`). There's also ``UIO_MEM_VIRTUAL`` for
280   virtual memory.
281
282-  ``phys_addr_t addr``: Required if the mapping is used. Fill in the
283   address of your memory block. This address is the one that appears in
284   sysfs.
285
286-  ``resource_size_t size``: Fill in the size of the memory block that
287   ``addr`` points to. If ``size`` is zero, the mapping is considered
288   unused. Note that you *must* initialize ``size`` with zero for all
289   unused mappings.
290
291-  ``void *internal_addr``: If you have to access this memory region
292   from within your kernel module, you will want to map it internally by
293   using something like :c:func:`ioremap()`. Addresses returned by
294   this function cannot be mapped to user space, so you must not store
295   it in ``addr``. Use ``internal_addr`` instead to remember such an
296   address.
297
298Please do not touch the ``map`` element of ``struct uio_mem``! It is
299used by the UIO framework to set up sysfs files for this mapping. Simply
300leave it alone.
301
302Sometimes, your device can have one or more port regions which can not
303be mapped to userspace. But if there are other possibilities for
304userspace to access these ports, it makes sense to make information
305about the ports available in sysfs. For each region, you have to set up
306a ``struct uio_port`` in the ``port[]`` array. Here's a description of
307the fields of ``struct uio_port``:
308
309-  ``char *porttype``: Required. Set this to one of the predefined
310   constants. Use ``UIO_PORT_X86`` for the ioports found in x86
311   architectures.
312
313-  ``unsigned long start``: Required if the port region is used. Fill in
314   the number of the first port of this region.
315
316-  ``unsigned long size``: Fill in the number of ports in this region.
317   If ``size`` is zero, the region is considered unused. Note that you
318   *must* initialize ``size`` with zero for all unused regions.
319
320Please do not touch the ``portio`` element of ``struct uio_port``! It is
321used internally by the UIO framework to set up sysfs files for this
322region. Simply leave it alone.
323
324Adding an interrupt handler
325---------------------------
326
327What you need to do in your interrupt handler depends on your hardware
328and on how you want to handle it. You should try to keep the amount of
329code in your kernel interrupt handler low. If your hardware requires no
330action that you *have* to perform after each interrupt, then your
331handler can be empty.
332
333If, on the other hand, your hardware *needs* some action to be performed
334after each interrupt, then you *must* do it in your kernel module. Note
335that you cannot rely on the userspace part of your driver. Your
336userspace program can terminate at any time, possibly leaving your
337hardware in a state where proper interrupt handling is still required.
338
339There might also be applications where you want to read data from your
340hardware at each interrupt and buffer it in a piece of kernel memory
341you've allocated for that purpose. With this technique you could avoid
342loss of data if your userspace program misses an interrupt.
343
344A note on shared interrupts: Your driver should support interrupt
345sharing whenever this is possible. It is possible if and only if your
346driver can detect whether your hardware has triggered the interrupt or
347not. This is usually done by looking at an interrupt status register. If
348your driver sees that the IRQ bit is actually set, it will perform its
349actions, and the handler returns IRQ_HANDLED. If the driver detects
350that it was not your hardware that caused the interrupt, it will do
351nothing and return IRQ_NONE, allowing the kernel to call the next
352possible interrupt handler.
353
354If you decide not to support shared interrupts, your card won't work in
355computers with no free interrupts. As this frequently happens on the PC
356platform, you can save yourself a lot of trouble by supporting interrupt
357sharing.
358
359Using uio_pdrv for platform devices
360-----------------------------------
361
362In many cases, UIO drivers for platform devices can be handled in a
363generic way. In the same place where you define your
364``struct platform_device``, you simply also implement your interrupt
365handler and fill your ``struct uio_info``. A pointer to this
366``struct uio_info`` is then used as ``platform_data`` for your platform
367device.
368
369You also need to set up an array of ``struct resource`` containing
370addresses and sizes of your memory mappings. This information is passed
371to the driver using the ``.resource`` and ``.num_resources`` elements of
372``struct platform_device``.
373
374You now have to set the ``.name`` element of ``struct platform_device``
375to ``"uio_pdrv"`` to use the generic UIO platform device driver. This
376driver will fill the ``mem[]`` array according to the resources given,
377and register the device.
378
379The advantage of this approach is that you only have to edit a file you
380need to edit anyway. You do not have to create an extra driver.
381
382Using uio_pdrv_genirq for platform devices
383------------------------------------------
384
385Especially in embedded devices, you frequently find chips where the irq
386pin is tied to its own dedicated interrupt line. In such cases, where
387you can be really sure the interrupt is not shared, we can take the
388concept of ``uio_pdrv`` one step further and use a generic interrupt
389handler. That's what ``uio_pdrv_genirq`` does.
390
391The setup for this driver is the same as described above for
392``uio_pdrv``, except that you do not implement an interrupt handler. The
393``.handler`` element of ``struct uio_info`` must remain ``NULL``. The
394``.irq_flags`` element must not contain ``IRQF_SHARED``.
395
396You will set the ``.name`` element of ``struct platform_device`` to
397``"uio_pdrv_genirq"`` to use this driver.
398
399The generic interrupt handler of ``uio_pdrv_genirq`` will simply disable
400the interrupt line using :c:func:`disable_irq_nosync()`. After
401doing its work, userspace can reenable the interrupt by writing
4020x00000001 to the UIO device file. The driver already implements an
403:c:func:`irq_control()` to make this possible, you must not
404implement your own.
405
406Using ``uio_pdrv_genirq`` not only saves a few lines of interrupt
407handler code. You also do not need to know anything about the chip's
408internal registers to create the kernel part of the driver. All you need
409to know is the irq number of the pin the chip is connected to.
410
411Using uio_dmem_genirq for platform devices
412------------------------------------------
413
414In addition to statically allocated memory ranges, they may also be a
415desire to use dynamically allocated regions in a user space driver. In
416particular, being able to access memory made available through the
417dma-mapping API, may be particularly useful. The ``uio_dmem_genirq``
418driver provides a way to accomplish this.
419
420This driver is used in a similar manner to the ``"uio_pdrv_genirq"``
421driver with respect to interrupt configuration and handling.
422
423Set the ``.name`` element of ``struct platform_device`` to
424``"uio_dmem_genirq"`` to use this driver.
425
426When using this driver, fill in the ``.platform_data`` element of
427``struct platform_device``, which is of type
428``struct uio_dmem_genirq_pdata`` and which contains the following
429elements:
430
431-  ``struct uio_info uioinfo``: The same structure used as the
432   ``uio_pdrv_genirq`` platform data
433
434-  ``unsigned int *dynamic_region_sizes``: Pointer to list of sizes of
435   dynamic memory regions to be mapped into user space.
436
437-  ``unsigned int num_dynamic_regions``: Number of elements in
438   ``dynamic_region_sizes`` array.
439
440The dynamic regions defined in the platform data will be appended to the
441`` mem[] `` array after the platform device resources, which implies
442that the total number of static and dynamic memory regions cannot exceed
443``MAX_UIO_MAPS``.
444
445The dynamic memory regions will be allocated when the UIO device file,
446``/dev/uioX`` is opened. Similar to static memory resources, the memory
447region information for dynamic regions is then visible via sysfs at
448``/sys/class/uio/uioX/maps/mapY/*``. The dynamic memory regions will be
449freed when the UIO device file is closed. When no processes are holding
450the device file open, the address returned to userspace is ~0.
451
452Writing a driver in userspace
453=============================
454
455Once you have a working kernel module for your hardware, you can write
456the userspace part of your driver. You don't need any special libraries,
457your driver can be written in any reasonable language, you can use
458floating point numbers and so on. In short, you can use all the tools
459and libraries you'd normally use for writing a userspace application.
460
461Getting information about your UIO device
462-----------------------------------------
463
464Information about all UIO devices is available in sysfs. The first thing
465you should do in your driver is check ``name`` and ``version`` to make
466sure you're talking to the right device and that its kernel driver has
467the version you expect.
468
469You should also make sure that the memory mapping you need exists and
470has the size you expect.
471
472There is a tool called ``lsuio`` that lists UIO devices and their
473attributes. It is available here:
474
475http://www.osadl.org/projects/downloads/UIO/user/
476
477With ``lsuio`` you can quickly check if your kernel module is loaded and
478which attributes it exports. Have a look at the manpage for details.
479
480The source code of ``lsuio`` can serve as an example for getting
481information about an UIO device. The file ``uio_helper.c`` contains a
482lot of functions you could use in your userspace driver code.
483
484mmap() device memory
485--------------------
486
487After you made sure you've got the right device with the memory mappings
488you need, all you have to do is to call :c:func:`mmap()` to map the
489device's memory to userspace.
490
491The parameter ``offset`` of the :c:func:`mmap()` call has a special
492meaning for UIO devices: It is used to select which mapping of your
493device you want to map. To map the memory of mapping N, you have to use
494N times the page size as your offset::
495
496        offset = N * getpagesize();
497
498N starts from zero, so if you've got only one memory range to map, set
499``offset = 0``. A drawback of this technique is that memory is always
500mapped beginning with its start address.
501
502Waiting for interrupts
503----------------------
504
505After you successfully mapped your devices memory, you can access it
506like an ordinary array. Usually, you will perform some initialization.
507After that, your hardware starts working and will generate an interrupt
508as soon as it's finished, has some data available, or needs your
509attention because an error occurred.
510
511``/dev/uioX`` is a read-only file. A :c:func:`read()` will always
512block until an interrupt occurs. There is only one legal value for the
513``count`` parameter of :c:func:`read()`, and that is the size of a
514signed 32 bit integer (4). Any other value for ``count`` causes
515:c:func:`read()` to fail. The signed 32 bit integer read is the
516interrupt count of your device. If the value is one more than the value
517you read the last time, everything is OK. If the difference is greater
518than one, you missed interrupts.
519
520You can also use :c:func:`select()` on ``/dev/uioX``.
521
522Generic PCI UIO driver
523======================
524
525The generic driver is a kernel module named uio_pci_generic. It can
526work with any device compliant to PCI 2.3 (circa 2002) and any compliant
527PCI Express device. Using this, you only need to write the userspace
528driver, removing the need to write a hardware-specific kernel module.
529
530Making the driver recognize the device
531--------------------------------------
532
533Since the driver does not declare any device ids, it will not get loaded
534automatically and will not automatically bind to any devices, you must
535load it and allocate id to the driver yourself. For example::
536
537     modprobe uio_pci_generic
538     echo "8086 10f5" > /sys/bus/pci/drivers/uio_pci_generic/new_id
539
540If there already is a hardware specific kernel driver for your device,
541the generic driver still won't bind to it, in this case if you want to
542use the generic driver (why would you?) you'll have to manually unbind
543the hardware specific driver and bind the generic driver, like this::
544
545        echo -n 0000:00:19.0 > /sys/bus/pci/drivers/e1000e/unbind
546        echo -n 0000:00:19.0 > /sys/bus/pci/drivers/uio_pci_generic/bind
547
548You can verify that the device has been bound to the driver by looking
549for it in sysfs, for example like the following::
550
551        ls -l /sys/bus/pci/devices/0000:00:19.0/driver
552
553Which if successful should print::
554
555      .../0000:00:19.0/driver -> ../../../bus/pci/drivers/uio_pci_generic
556
557Note that the generic driver will not bind to old PCI 2.2 devices. If
558binding the device failed, run the following command::
559
560      dmesg
561
562and look in the output for failure reasons.
563
564Things to know about uio_pci_generic
565------------------------------------
566
567Interrupts are handled using the Interrupt Disable bit in the PCI
568command register and Interrupt Status bit in the PCI status register.
569All devices compliant to PCI 2.3 (circa 2002) and all compliant PCI
570Express devices should support these bits. uio_pci_generic detects
571this support, and won't bind to devices which do not support the
572Interrupt Disable Bit in the command register.
573
574On each interrupt, uio_pci_generic sets the Interrupt Disable bit.
575This prevents the device from generating further interrupts until the
576bit is cleared. The userspace driver should clear this bit before
577blocking and waiting for more interrupts.
578
579Writing userspace driver using uio_pci_generic
580------------------------------------------------
581
582Userspace driver can use pci sysfs interface, or the libpci library that
583wraps it, to talk to the device and to re-enable interrupts by writing
584to the command register.
585
586Example code using uio_pci_generic
587----------------------------------
588
589Here is some sample userspace driver code using uio_pci_generic::
590
591    #include <stdlib.h>
592    #include <stdio.h>
593    #include <unistd.h>
594    #include <sys/types.h>
595    #include <sys/stat.h>
596    #include <fcntl.h>
597    #include <errno.h>
598
599    int main()
600    {
601        int uiofd;
602        int configfd;
603        int err;
604        int i;
605        unsigned icount;
606        unsigned char command_high;
607
608        uiofd = open("/dev/uio0", O_RDONLY);
609        if (uiofd < 0) {
610            perror("uio open:");
611            return errno;
612        }
613        configfd = open("/sys/class/uio/uio0/device/config", O_RDWR);
614        if (configfd < 0) {
615            perror("config open:");
616            return errno;
617        }
618
619        /* Read and cache command value */
620        err = pread(configfd, &command_high, 1, 5);
621        if (err != 1) {
622            perror("command config read:");
623            return errno;
624        }
625        command_high &= ~0x4;
626
627        for(i = 0;; ++i) {
628            /* Print out a message, for debugging. */
629            if (i == 0)
630                fprintf(stderr, "Started uio test driver.\n");
631            else
632                fprintf(stderr, "Interrupts: %d\n", icount);
633
634            /****************************************/
635            /* Here we got an interrupt from the
636               device. Do something to it. */
637            /****************************************/
638
639            /* Re-enable interrupts. */
640            err = pwrite(configfd, &command_high, 1, 5);
641            if (err != 1) {
642                perror("config write:");
643                break;
644            }
645
646            /* Wait for next interrupt. */
647            err = read(uiofd, &icount, 4);
648            if (err != 4) {
649                perror("uio read:");
650                break;
651            }
652
653        }
654        return errno;
655    }
656
657Generic Hyper-V UIO driver
658==========================
659
660The generic driver is a kernel module named uio_hv_generic. It
661supports devices on the Hyper-V VMBus similar to uio_pci_generic on
662PCI bus.
663
664Making the driver recognize the device
665--------------------------------------
666
667Since the driver does not declare any device GUID's, it will not get
668loaded automatically and will not automatically bind to any devices, you
669must load it and allocate id to the driver yourself. For example, to use
670the network device class GUID::
671
672     modprobe uio_hv_generic
673     echo "f8615163-df3e-46c5-913f-f2d2f965ed0e" > /sys/bus/vmbus/drivers/uio_hv_generic/new_id
674
675If there already is a hardware specific kernel driver for the device,
676the generic driver still won't bind to it, in this case if you want to
677use the generic driver for a userspace library you'll have to manually unbind
678the hardware specific driver and bind the generic driver, using the device specific GUID
679like this::
680
681          echo -n ed963694-e847-4b2a-85af-bc9cfc11d6f3 > /sys/bus/vmbus/drivers/hv_netvsc/unbind
682          echo -n ed963694-e847-4b2a-85af-bc9cfc11d6f3 > /sys/bus/vmbus/drivers/uio_hv_generic/bind
683
684You can verify that the device has been bound to the driver by looking
685for it in sysfs, for example like the following::
686
687        ls -l /sys/bus/vmbus/devices/ed963694-e847-4b2a-85af-bc9cfc11d6f3/driver
688
689Which if successful should print::
690
691      .../ed963694-e847-4b2a-85af-bc9cfc11d6f3/driver -> ../../../bus/vmbus/drivers/uio_hv_generic
692
693Things to know about uio_hv_generic
694-----------------------------------
695
696On each interrupt, uio_hv_generic sets the Interrupt Disable bit. This
697prevents the device from generating further interrupts until the bit is
698cleared. The userspace driver should clear this bit before blocking and
699waiting for more interrupts.
700
701When host rescinds a device, the interrupt file descriptor is marked down
702and any reads of the interrupt file descriptor will return -EIO. Similar
703to a closed socket or disconnected serial device.
704
705The vmbus device regions are mapped into uio device resources:
706    0) Channel ring buffers: guest to host and host to guest
707    1) Guest to host interrupt signalling pages
708    2) Guest to host monitor page
709    3) Network receive buffer region
710    4) Network send buffer region
711
712If a subchannel is created by a request to host, then the uio_hv_generic
713device driver will create a sysfs binary file for the per-channel ring buffer.
714For example::
715
716	/sys/bus/vmbus/devices/3811fe4d-0fa0-4b62-981a-74fc1084c757/channels/21/ring
717
718Further information
719===================
720
721-  `OSADL homepage. <http://www.osadl.org>`_
722
723-  `Linutronix homepage. <http://www.linutronix.de>`_
724