xref: /openbmc/linux/arch/xtensa/Kconfig (revision 29c37341)

# SPDX-License-Identifier: GPL-2.0
config XTENSA
	def_bool y
	select ARCH_32BIT_OFF_T
	select ARCH_HAS_BINFMT_FLAT if !MMU
	select ARCH_HAS_DMA_PREP_COHERENT if MMU
	select ARCH_HAS_SYNC_DMA_FOR_CPU if MMU
	select ARCH_HAS_SYNC_DMA_FOR_DEVICE if MMU
	select ARCH_HAS_DMA_SET_UNCACHED if MMU
	select ARCH_USE_QUEUED_RWLOCKS
	select ARCH_USE_QUEUED_SPINLOCKS
	select ARCH_WANT_FRAME_POINTERS
	select ARCH_WANT_IPC_PARSE_VERSION
	select BUILDTIME_TABLE_SORT
	select CLONE_BACKWARDS
	select COMMON_CLK
	select DMA_REMAP if MMU
	select GENERIC_ATOMIC64
	select GENERIC_CLOCKEVENTS
	select GENERIC_IRQ_SHOW
	select GENERIC_PCI_IOMAP
	select GENERIC_SCHED_CLOCK
	select GENERIC_STRNCPY_FROM_USER if KASAN
	select HAVE_ARCH_AUDITSYSCALL
	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL
	select HAVE_ARCH_KASAN if MMU && !XIP_KERNEL
	select HAVE_ARCH_SECCOMP_FILTER
	select HAVE_ARCH_TRACEHOOK
	select HAVE_DEBUG_KMEMLEAK
	select HAVE_DMA_CONTIGUOUS
	select HAVE_EXIT_THREAD
	select HAVE_FUNCTION_TRACER
	select HAVE_FUTEX_CMPXCHG if !MMU
	select HAVE_HW_BREAKPOINT if PERF_EVENTS
	select HAVE_IRQ_TIME_ACCOUNTING
	select HAVE_OPROFILE
	select HAVE_PCI
	select HAVE_PERF_EVENTS
	select HAVE_STACKPROTECTOR
	select HAVE_SYSCALL_TRACEPOINTS
	select IRQ_DOMAIN
	select MODULES_USE_ELF_RELA
	select PERF_USE_VMALLOC
	select VIRT_TO_BUS
	help
	  Xtensa processors are 32-bit RISC machines designed by Tensilica
	  primarily for embedded systems.  These processors are both
	  configurable and extensible.  The Linux port to the Xtensa
	  architecture supports all processor configurations and extensions,
	  with reasonable minimum requirements.  The Xtensa Linux project has
	  a home page at <http://www.linux-xtensa.org/>.

config GENERIC_HWEIGHT
	def_bool y

config ARCH_HAS_ILOG2_U32
	def_bool n

config ARCH_HAS_ILOG2_U64
	def_bool n

config NO_IOPORT_MAP
	def_bool n

config HZ
	int
	default 100

config LOCKDEP_SUPPORT
	def_bool y

config STACKTRACE_SUPPORT
	def_bool y

config TRACE_IRQFLAGS_SUPPORT
	def_bool y

config MMU
	def_bool n

config HAVE_XTENSA_GPIO32
	def_bool n

config KASAN_SHADOW_OFFSET
	hex
	default 0x6e400000

menu "Processor type and features"

choice
	prompt "Xtensa Processor Configuration"
	default XTENSA_VARIANT_FSF

config XTENSA_VARIANT_FSF
	bool "fsf - default (not generic) configuration"
	select MMU

config XTENSA_VARIANT_DC232B
	bool "dc232b - Diamond 232L Standard Core Rev.B (LE)"
	select MMU
	select HAVE_XTENSA_GPIO32
	help
	  This variant refers to Tensilica's Diamond 232L Standard core Rev.B (LE).

config XTENSA_VARIANT_DC233C
	bool "dc233c - Diamond 233L Standard Core Rev.C (LE)"
	select MMU
	select HAVE_XTENSA_GPIO32
	help
	  This variant refers to Tensilica's Diamond 233L Standard core Rev.C (LE).

config XTENSA_VARIANT_CUSTOM
	bool "Custom Xtensa processor configuration"
	select HAVE_XTENSA_GPIO32
	help
	  Select this variant to use a custom Xtensa processor configuration.
	  You will be prompted for a processor variant CORENAME.
endchoice

config XTENSA_VARIANT_CUSTOM_NAME
	string "Xtensa Processor Custom Core Variant Name"
	depends on XTENSA_VARIANT_CUSTOM
	help
	  Provide the name of a custom Xtensa processor variant.
	  This CORENAME selects arch/xtensa/variant/CORENAME.
	  Don't forget you have to select MMU if you have one.

config XTENSA_VARIANT_NAME
	string
	default "dc232b"			if XTENSA_VARIANT_DC232B
	default "dc233c"			if XTENSA_VARIANT_DC233C
	default "fsf"				if XTENSA_VARIANT_FSF
	default XTENSA_VARIANT_CUSTOM_NAME	if XTENSA_VARIANT_CUSTOM

config XTENSA_VARIANT_MMU
	bool "Core variant has a Full MMU (TLB, Pages, Protection, etc)"
	depends on XTENSA_VARIANT_CUSTOM
	default y
	select MMU
	help
	  Build a Conventional Kernel with full MMU support,
	  ie: it supports a TLB with auto-loading, page protection.

config XTENSA_VARIANT_HAVE_PERF_EVENTS
	bool "Core variant has Performance Monitor Module"
	depends on XTENSA_VARIANT_CUSTOM
	default n
	help
	  Enable if core variant has Performance Monitor Module with
	  External Registers Interface.

	  If unsure, say N.

config XTENSA_FAKE_NMI
	bool "Treat PMM IRQ as NMI"
	depends on XTENSA_VARIANT_HAVE_PERF_EVENTS
	default n
	help
	  If PMM IRQ is the only IRQ at EXCM level it is safe to
	  treat it as NMI, which improves accuracy of profiling.

	  If there are other interrupts at or above PMM IRQ priority level
	  but not above the EXCM level, PMM IRQ still may be treated as NMI,
	  but only if these IRQs are not used. There will be a build warning
	  saying that this is not safe, and a bugcheck if one of these IRQs
	  actually fire.

	  If unsure, say N.

config XTENSA_UNALIGNED_USER
	bool "Unaligned memory access in user space"
	help
	  The Xtensa architecture currently does not handle unaligned
	  memory accesses in hardware but through an exception handler.
	  Per default, unaligned memory accesses are disabled in user space.

	  Say Y here to enable unaligned memory access in user space.

config HAVE_SMP
	bool "System Supports SMP (MX)"
	depends on XTENSA_VARIANT_CUSTOM
	select XTENSA_MX
	help
	  This option is used to indicate that the system-on-a-chip (SOC)
	  supports Multiprocessing. Multiprocessor support implemented above
	  the CPU core definition and currently needs to be selected manually.

	  Multiprocessor support is implemented with external cache and
	  interrupt controllers.

	  The MX interrupt distributer adds Interprocessor Interrupts
	  and causes the IRQ numbers to be increased by 4 for devices
	  like the open cores ethernet driver and the serial interface.

	  You still have to select "Enable SMP" to enable SMP on this SOC.

config SMP
	bool "Enable Symmetric multi-processing support"
	depends on HAVE_SMP
	select GENERIC_SMP_IDLE_THREAD
	help
	  Enabled SMP Software; allows more than one CPU/CORE
	  to be activated during startup.

config NR_CPUS
	depends on SMP
	int "Maximum number of CPUs (2-32)"
	range 2 32
	default "4"

config HOTPLUG_CPU
	bool "Enable CPU hotplug support"
	depends on SMP
	help
	  Say Y here to allow turning CPUs off and on. CPUs can be
	  controlled through /sys/devices/system/cpu.

	  Say N if you want to disable CPU hotplug.

config SECCOMP
	bool
	prompt "Enable seccomp to safely compute untrusted bytecode"
	help
	  This kernel feature is useful for number crunching applications
	  that may need to compute untrusted bytecode during their
	  execution. By using pipes or other transports made available to
	  the process as file descriptors supporting the read/write
	  syscalls, it's possible to isolate those applications in
	  their own address space using seccomp. Once seccomp is
	  enabled via prctl(PR_SET_SECCOMP), it cannot be disabled
	  and the task is only allowed to execute a few safe syscalls
	  defined by each seccomp mode.

config FAST_SYSCALL_XTENSA
	bool "Enable fast atomic syscalls"
	default n
	help
	  fast_syscall_xtensa is a syscall that can make atomic operations
	  on UP kernel when processor has no s32c1i support.

	  This syscall is deprecated. It may have issues when called with
	  invalid arguments. It is provided only for backwards compatibility.
	  Only enable it if your userspace software requires it.

	  If unsure, say N.

config FAST_SYSCALL_SPILL_REGISTERS
	bool "Enable spill registers syscall"
	default n
	help
	  fast_syscall_spill_registers is a syscall that spills all active
	  register windows of a calling userspace task onto its stack.

	  This syscall is deprecated. It may have issues when called with
	  invalid arguments. It is provided only for backwards compatibility.
	  Only enable it if your userspace software requires it.

	  If unsure, say N.

config USER_ABI_CALL0
	bool

choice
	prompt "Userspace ABI"
	default USER_ABI_DEFAULT
	help
	  Select supported userspace ABI.

	  If unsure, choose the default ABI.

config USER_ABI_DEFAULT
	bool "Default ABI only"
	help
	  Assume default userspace ABI. For XEA2 cores it is windowed ABI.
	  call0 ABI binaries may be run on such kernel, but signal delivery
	  will not work correctly for them.

config USER_ABI_CALL0_ONLY
	bool "Call0 ABI only"
	select USER_ABI_CALL0
	help
	  Select this option to support only call0 ABI in userspace.
	  Windowed ABI binaries will crash with a segfault caused by
	  an illegal instruction exception on the first 'entry' opcode.

	  Choose this option if you're planning to run only user code
	  built with call0 ABI.

config USER_ABI_CALL0_PROBE
	bool "Support both windowed and call0 ABI by probing"
	select USER_ABI_CALL0
	help
	  Select this option to support both windowed and call0 userspace
	  ABIs. When enabled all processes are started with PS.WOE disabled
	  and a fast user exception handler for an illegal instruction is
	  used to turn on PS.WOE bit on the first 'entry' opcode executed by
	  the userspace.

	  This option should be enabled for the kernel that must support
	  both call0 and windowed ABIs in userspace at the same time.

	  Note that Xtensa ISA does not guarantee that entry opcode will
	  raise an illegal instruction exception on cores with XEA2 when
	  PS.WOE is disabled, check whether the target core supports it.

endchoice

endmenu

config XTENSA_CALIBRATE_CCOUNT
	def_bool n
	help
	  On some platforms (XT2000, for example), the CPU clock rate can
	  vary.  The frequency can be determined, however, by measuring
	  against a well known, fixed frequency, such as an UART oscillator.

config SERIAL_CONSOLE
	def_bool n

config PLATFORM_HAVE_XIP
	def_bool n

menu "Platform options"

choice
	prompt "Xtensa System Type"
	default XTENSA_PLATFORM_ISS

config XTENSA_PLATFORM_ISS
	bool "ISS"
	select XTENSA_CALIBRATE_CCOUNT
	select SERIAL_CONSOLE
	help
	  ISS is an acronym for Tensilica's Instruction Set Simulator.

config XTENSA_PLATFORM_XT2000
	bool "XT2000"
	select HAVE_IDE
	help
	  XT2000 is the name of Tensilica's feature-rich emulation platform.
	  This hardware is capable of running a full Linux distribution.

config XTENSA_PLATFORM_XTFPGA
	bool "XTFPGA"
	select ETHOC if ETHERNET
	select PLATFORM_WANT_DEFAULT_MEM if !MMU
	select SERIAL_CONSOLE
	select XTENSA_CALIBRATE_CCOUNT
	select PLATFORM_HAVE_XIP
	help
	  XTFPGA is the name of Tensilica board family (LX60, LX110, LX200, ML605).
	  This hardware is capable of running a full Linux distribution.

endchoice

config PLATFORM_NR_IRQS
	int
	default 3 if XTENSA_PLATFORM_XT2000
	default 0

config XTENSA_CPU_CLOCK
	int "CPU clock rate [MHz]"
	depends on !XTENSA_CALIBRATE_CCOUNT
	default 16

config GENERIC_CALIBRATE_DELAY
	bool "Auto calibration of the BogoMIPS value"
	help
	  The BogoMIPS value can easily be derived from the CPU frequency.

config CMDLINE_BOOL
	bool "Default bootloader kernel arguments"

config CMDLINE
	string "Initial kernel command string"
	depends on CMDLINE_BOOL
	default "console=ttyS0,38400 root=/dev/ram"
	help
	  On some architectures (EBSA110 and CATS), there is currently no way
	  for the boot loader to pass arguments to the kernel. For these
	  architectures, you should supply some command-line options at build
	  time by entering them here. As a minimum, you should specify the
	  memory size and the root device (e.g., mem=64M root=/dev/nfs).

config USE_OF
	bool "Flattened Device Tree support"
	select OF
	select OF_EARLY_FLATTREE
	help
	  Include support for flattened device tree machine descriptions.

config BUILTIN_DTB_SOURCE
	string "DTB to build into the kernel image"
	depends on OF

config PARSE_BOOTPARAM
	bool "Parse bootparam block"
	default y
	help
	  Parse parameters passed to the kernel from the bootloader. It may
	  be disabled if the kernel is known to run without the bootloader.

	  If unsure, say Y.

config BLK_DEV_SIMDISK
	tristate "Host file-based simulated block device support"
	default n
	depends on XTENSA_PLATFORM_ISS && BLOCK
	help
	  Create block devices that map to files in the host file system.
	  Device binding to host file may be changed at runtime via proc
	  interface provided the device is not in use.

config BLK_DEV_SIMDISK_COUNT
	int "Number of host file-based simulated block devices"
	range 1 10
	depends on BLK_DEV_SIMDISK
	default 2
	help
	  This is the default minimal number of created block devices.
	  Kernel/module parameter 'simdisk_count' may be used to change this
	  value at runtime. More file names (but no more than 10) may be
	  specified as parameters, simdisk_count grows accordingly.

config SIMDISK0_FILENAME
	string "Host filename for the first simulated device"
	depends on BLK_DEV_SIMDISK = y
	default ""
	help
	  Attach a first simdisk to a host file. Conventionally, this file
	  contains a root file system.

config SIMDISK1_FILENAME
	string "Host filename for the second simulated device"
	depends on BLK_DEV_SIMDISK = y && BLK_DEV_SIMDISK_COUNT != 1
	default ""
	help
	  Another simulated disk in a host file for a buildroot-independent
	  storage.

config XTFPGA_LCD
	bool "Enable XTFPGA LCD driver"
	depends on XTENSA_PLATFORM_XTFPGA
	default n
	help
	  There's a 2x16 LCD on most of XTFPGA boards, kernel may output
	  progress messages there during bootup/shutdown. It may be useful
	  during board bringup.

	  If unsure, say N.

config XTFPGA_LCD_BASE_ADDR
	hex "XTFPGA LCD base address"
	depends on XTFPGA_LCD
	default "0x0d0c0000"
	help
	  Base address of the LCD controller inside KIO region.
	  Different boards from XTFPGA family have LCD controller at different
	  addresses. Please consult prototyping user guide for your board for
	  the correct address. Wrong address here may lead to hardware lockup.

config XTFPGA_LCD_8BIT_ACCESS
	bool "Use 8-bit access to XTFPGA LCD"
	depends on XTFPGA_LCD
	default n
	help
	  LCD may be connected with 4- or 8-bit interface, 8-bit access may
	  only be used with 8-bit interface. Please consult prototyping user
	  guide for your board for the correct interface width.

comment "Kernel memory layout"

config INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX
	bool "Initialize Xtensa MMU inside the Linux kernel code"
	depends on !XTENSA_VARIANT_FSF && !XTENSA_VARIANT_DC232B
	default y if XTENSA_VARIANT_DC233C || XTENSA_VARIANT_CUSTOM
	help
	  Earlier version initialized the MMU in the exception vector
	  before jumping to _startup in head.S and had an advantage that
	  it was possible to place a software breakpoint at 'reset' and
	  then enter your normal kernel breakpoints once the MMU was mapped
	  to the kernel mappings (0XC0000000).

	  This unfortunately won't work for U-Boot and likely also wont
	  work for using KEXEC to have a hot kernel ready for doing a
	  KDUMP.

	  So now the MMU is initialized in head.S but it's necessary to
	  use hardware breakpoints (gdb 'hbreak' cmd) to break at _startup.
	  xt-gdb can't place a Software Breakpoint in the  0XD region prior
	  to mapping the MMU and after mapping even if the area of low memory
	  was mapped gdb wouldn't remove the breakpoint on hitting it as the
	  PC wouldn't match. Since Hardware Breakpoints are recommended for
	  Linux configurations it seems reasonable to just assume they exist
	  and leave this older mechanism for unfortunate souls that choose
	  not to follow Tensilica's recommendation.

	  Selecting this will cause U-Boot to set the KERNEL Load and Entry
	  address at 0x00003000 instead of the mapped std of 0xD0003000.

	  If in doubt, say Y.

config XIP_KERNEL
	bool "Kernel Execute-In-Place from ROM"
	depends on PLATFORM_HAVE_XIP
	help
	  Execute-In-Place allows the kernel to run from non-volatile storage
	  directly addressable by the CPU, such as NOR flash. This saves RAM
	  space since the text section of the kernel is not loaded from flash
	  to RAM. Read-write sections, such as the data section and stack,
	  are still copied to RAM. The XIP kernel is not compressed since
	  it has to run directly from flash, so it will take more space to
	  store it. The flash address used to link the kernel object files,
	  and for storing it, is configuration dependent. Therefore, if you
	  say Y here, you must know the proper physical address where to
	  store the kernel image depending on your own flash memory usage.

	  Also note that the make target becomes "make xipImage" rather than
	  "make Image" or "make uImage". The final kernel binary to put in
	  ROM memory will be arch/xtensa/boot/xipImage.

	  If unsure, say N.

config MEMMAP_CACHEATTR
	hex "Cache attributes for the memory address space"
	depends on !MMU
	default 0x22222222
	help
	  These cache attributes are set up for noMMU systems. Each hex digit
	  specifies cache attributes for the corresponding 512MB memory
	  region: bits 0..3 -- for addresses 0x00000000..0x1fffffff,
	  bits 4..7 -- for addresses 0x20000000..0x3fffffff, and so on.

	  Cache attribute values are specific for the MMU type.
	  For region protection MMUs:
	    1: WT cached,
	    2: cache bypass,
	    4: WB cached,
	    f: illegal.
	  For ful MMU:
	    bit 0: executable,
	    bit 1: writable,
	    bits 2..3:
	      0: cache bypass,
	      1: WB cache,
	      2: WT cache,
	      3: special (c and e are illegal, f is reserved).
	  For MPU:
	    0: illegal,
	    1: WB cache,
	    2: WB, no-write-allocate cache,
	    3: WT cache,
	    4: cache bypass.

config KSEG_PADDR
	hex "Physical address of the KSEG mapping"
	depends on INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX && MMU
	default 0x00000000
	help
	  This is the physical address where KSEG is mapped. Please refer to
	  the chosen KSEG layout help for the required address alignment.
	  Unpacked kernel image (including vectors) must be located completely
	  within KSEG.
	  Physical memory below this address is not available to linux.

	  If unsure, leave the default value here.

config KERNEL_VIRTUAL_ADDRESS
	hex "Kernel virtual address"
	depends on MMU && XIP_KERNEL
	default 0xd0003000
	help
	  This is the virtual address where the XIP kernel is mapped.
	  XIP kernel may be mapped into KSEG or KIO region, virtual address
	  provided here must match kernel load address provided in
	  KERNEL_LOAD_ADDRESS.

config KERNEL_LOAD_ADDRESS
	hex "Kernel load address"
	default 0x60003000 if !MMU
	default 0x00003000 if MMU && INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX
	default 0xd0003000 if MMU && !INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX
	help
	  This is the address where the kernel is loaded.
	  It is virtual address for MMUv2 configurations and physical address
	  for all other configurations.

	  If unsure, leave the default value here.

choice
	prompt "Relocatable vectors location"
	default XTENSA_VECTORS_IN_TEXT
	help
	  Choose whether relocatable vectors are merged into the kernel .text
	  or placed separately at runtime. This option does not affect
	  configurations without VECBASE register where vectors are always
	  placed at their hardware-defined locations.

config XTENSA_VECTORS_IN_TEXT
	bool "Merge relocatable vectors into kernel text"
	depends on !MTD_XIP
	help
	  This option puts relocatable vectors into the kernel .text section
	  with proper alignment.
	  This is a safe choice for most configurations.

config XTENSA_VECTORS_SEPARATE
	bool "Put relocatable vectors at fixed address"
	help
	  This option puts relocatable vectors at specific virtual address.
	  Vectors are merged with the .init data in the kernel image and
	  are copied into their designated location during kernel startup.
	  Use it to put vectors into IRAM or out of FLASH on kernels with
	  XIP-aware MTD support.

endchoice

config VECTORS_ADDR
	hex "Kernel vectors virtual address"
	default 0x00000000
	depends on XTENSA_VECTORS_SEPARATE
	help
	  This is the virtual address of the (relocatable) vectors base.
	  It must be within KSEG if MMU is used.

config XIP_DATA_ADDR
	hex "XIP kernel data virtual address"
	depends on XIP_KERNEL
	default 0x00000000
	help
	  This is the virtual address where XIP kernel data is copied.
	  It must be within KSEG if MMU is used.

config PLATFORM_WANT_DEFAULT_MEM
	def_bool n

config DEFAULT_MEM_START
	hex
	prompt "PAGE_OFFSET/PHYS_OFFSET" if !MMU && PLATFORM_WANT_DEFAULT_MEM
	default 0x60000000 if PLATFORM_WANT_DEFAULT_MEM
	default 0x00000000
	help
	  This is the base address used for both PAGE_OFFSET and PHYS_OFFSET
	  in noMMU configurations.

	  If unsure, leave the default value here.

choice
	prompt "KSEG layout"
	depends on MMU
	default XTENSA_KSEG_MMU_V2

config XTENSA_KSEG_MMU_V2
	bool "MMUv2: 128MB cached + 128MB uncached"
	help
	  MMUv2 compatible kernel memory map: TLB way 5 maps 128MB starting
	  at KSEG_PADDR to 0xd0000000 with cache and to 0xd8000000
	  without cache.
	  KSEG_PADDR must be aligned to 128MB.

config XTENSA_KSEG_256M
	bool "256MB cached + 256MB uncached"
	depends on INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX
	help
	  TLB way 6 maps 256MB starting at KSEG_PADDR to 0xb0000000
	  with cache and to 0xc0000000 without cache.
	  KSEG_PADDR must be aligned to 256MB.

config XTENSA_KSEG_512M
	bool "512MB cached + 512MB uncached"
	depends on INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX
	help
	  TLB way 6 maps 512MB starting at KSEG_PADDR to 0xa0000000
	  with cache and to 0xc0000000 without cache.
	  KSEG_PADDR must be aligned to 256MB.

endchoice

config HIGHMEM
	bool "High Memory Support"
	depends on MMU
	help
	  Linux can use the full amount of RAM in the system by
	  default. However, the default MMUv2 setup only maps the
	  lowermost 128 MB of memory linearly to the areas starting
	  at 0xd0000000 (cached) and 0xd8000000 (uncached).
	  When there are more than 128 MB memory in the system not
	  all of it can be "permanently mapped" by the kernel.
	  The physical memory that's not permanently mapped is called
	  "high memory".

	  If you are compiling a kernel which will never run on a
	  machine with more than 128 MB total physical RAM, answer
	  N here.

	  If unsure, say Y.

config FORCE_MAX_ZONEORDER
	int "Maximum zone order"
	default "11"
	help
	  The kernel memory allocator divides physically contiguous memory
	  blocks into "zones", where each zone is a power of two number of
	  pages.  This option selects the largest power of two that the kernel
	  keeps in the memory allocator.  If you need to allocate very large
	  blocks of physically contiguous memory, then you may need to
	  increase this value.

	  This config option is actually maximum order plus one. For example,
	  a value of 11 means that the largest free memory block is 2^10 pages.

endmenu

menu "Power management options"

source "kernel/power/Kconfig"

endmenu