1=================
2SPI NOR framework
3=================
4
5Part I - Why do we need this framework?
6---------------------------------------
7
8SPI bus controllers (drivers/spi/) only deal with streams of bytes; the bus
9controller operates agnostic of the specific device attached. However, some
10controllers (such as Freescale's QuadSPI controller) cannot easily handle
11arbitrary streams of bytes, but rather are designed specifically for SPI NOR.
12
13In particular, Freescale's QuadSPI controller must know the NOR commands to
14find the right LUT sequence. Unfortunately, the SPI subsystem has no notion of
15opcodes, addresses, or data payloads; a SPI controller simply knows to send or
16receive bytes (Tx and Rx). Therefore, we must define a new layering scheme under
17which the controller driver is aware of the opcodes, addressing, and other
18details of the SPI NOR protocol.
19
20Part II - How does the framework work?
21--------------------------------------
22
23This framework just adds a new layer between the MTD and the SPI bus driver.
24With this new layer, the SPI NOR controller driver does not depend on the
25m25p80 code anymore.
26
27Before this framework, the layer is like::
28
29                   MTD
30         ------------------------
31                  m25p80
32         ------------------------
33	       SPI bus driver
34         ------------------------
35	        SPI NOR chip
36
37After this framework, the layer is like::
38
39                   MTD
40         ------------------------
41              SPI NOR framework
42         ------------------------
43                  m25p80
44         ------------------------
45	       SPI bus driver
46         ------------------------
47	       SPI NOR chip
48
49With the SPI NOR controller driver (Freescale QuadSPI), it looks like::
50
51                   MTD
52         ------------------------
53              SPI NOR framework
54         ------------------------
55                fsl-quadSPI
56         ------------------------
57	       SPI NOR chip
58
59Part III - How can drivers use the framework?
60---------------------------------------------
61
62The main API is spi_nor_scan(). Before you call the hook, a driver should
63initialize the necessary fields for spi_nor{}. Please see
64drivers/mtd/spi-nor/spi-nor.c for detail. Please also refer to spi-fsl-qspi.c
65when you want to write a new driver for a SPI NOR controller.
66Another API is spi_nor_restore(), this is used to restore the status of SPI
67flash chip such as addressing mode. Call it whenever detach the driver from
68device or reboot the system.
69