Documentation/frv/features.txt - arm/linux - Git at Google

 			 ===========================
 			 FUJITSU FR-V LINUX FEATURES
 			 ===========================

 This kernel port has a number of features of which the user should be aware:

  (*) Linux and uClinux

      The FR-V architecture port supports both normal MMU linux and uClinux out
      of the same sources.


  (*) CPU support

      Support for the FR401, FR403, FR405, FR451 and FR555 CPUs should work with
      the same uClinux kernel configuration.

      In normal (MMU) Linux mode, only the FR451 CPU will work as that is the
      only one with a suitably featured CPU.

      The kernel is written and compiled with the assumption that only the
      bottom 32 GR registers and no FR registers will be used by the kernel
      itself, however all extra userspace registers will be saved on context
      switch. Note that since most CPUs can't support lazy switching, no attempt
      is made to do lazy register saving where that would be possible (FR555
      only currently).


  (*) Board support

      The board on which the kernel will run can be configured on the "Processor
      type and features" configuration tab.

      Set the System to "MB93093-PDK" to boot from the MB93093 (FR403) PDK.

      Set the System to "MB93091-VDK" to boot from the CB11, CB30, CB41, CB60,
      CB70 or CB451 VDK boards. Set the Motherboard setting to "MB93090-MB00" to
      boot with the standard ATA90590B VDK motherboard, and set it to "None" to
      boot without any motherboard.


  (*) Binary Formats

      The only userspace binary format supported is FDPIC ELF. Normal ELF, FLAT
      and AOUT binaries are not supported for this architecture.

      FDPIC ELF supports shared library and program interpreter facilities.


  (*) Scheduler Speed

      The kernel scheduler runs at 100Hz irrespective of the clock speed on this
      architecture. This value is set in asm/param.h (see the HZ macro defined
      there).


  (*) Normal (MMU) Linux Memory Layout.

      See mmu-layout.txt in this directory for a description of the normal linux
      memory layout

      See include/asm-frv/mem-layout.h for constants pertaining to the memory
      layout.

      See include/asm-frv/mb-regs.h for the constants pertaining to the I/O bus
      controller configuration.


  (*) uClinux Memory Layout

      The memory layout used by the uClinux kernel is as follows:

 	0x00000000 - 0x00000FFF		Null pointer catch page
 	0x20000000 - 0x200FFFFF CS2#    [PDK] FPGA
 	0xC0000000 - 0xCFFFFFFF		SDRAM
 	0xC0000000			Base of Linux kernel image
 	0xE0000000 - 0xEFFFFFFF	CS2#	[VDK] SLBUS/PCI window
 	0xF0000000 - 0xF0FFFFFF	CS5#	MB93493 CSC area (DAV daughter board)
 	0xF1000000 - 0xF1FFFFFF	CS7#	[CB70/CB451] CPU-card PCMCIA port space
 	0xFC000000 - 0xFC0FFFFF	CS1#	[VDK] MB86943 config space
 	0xFC100000 - 0xFC1FFFFF	CS6#	[CB70/CB451] CPU-card DM9000 NIC space
 	0xFC100000 - 0xFC1FFFFF	CS6#	[PDK] AX88796 NIC space
 	0xFC200000 - 0xFC2FFFFF	CS3#	MB93493 CSR area (DAV daughter board)
 	0xFD000000 - 0xFDFFFFFF	CS4#	[CB70/CB451] CPU-card extra flash space
 	0xFE000000 - 0xFEFFFFFF		Internal CPU peripherals
 	0xFF000000 - 0xFF1FFFFF	CS0#	Flash 1
 	0xFF200000 - 0xFF3FFFFF	CS0#	Flash 2
 	0xFFC00000 - 0xFFC0001F	CS0#	[VDK] FPGA

      The kernel reads the size of the SDRAM from the memory bus controller
      registers by default.

      The kernel initialisation code (1) adjusts the SDRAM base addresses to
      move the SDRAM to desired address, (2) moves the kernel image down to the
      bottom of SDRAM, (3) adjusts the bus controller registers to move I/O
      windows, and (4) rearranges the protection registers to protect all of
      this.

      The reasons for doing this are: (1) the page at address 0 should be
      inaccessible so that NULL pointer errors can be caught; and (2) the bottom
      three quarters are left unoccupied so that an FR-V CPU with an MMU can use
      it for virtual userspace mappings.

      See include/asm-frv/mem-layout.h for constants pertaining to the memory
      layout.

      See include/asm-frv/mb-regs.h for the constants pertaining to the I/O bus
      controller configuration.


  (*) uClinux Memory Protection

      A DAMPR register is used to cover the entire region used for I/O
      (0xE0000000 - 0xFFFFFFFF). This permits the kernel to make uncached
      accesses to this region. Userspace is not permitted to access it.

      The DAMPR/IAMPR protection registers not in use for any other purpose are
      tiled over the top of the SDRAM such that:

 	(1) The core kernel image is covered by as small a tile as possible
             granting only the kernel access to the underlying data, whilst
             making sure no SDRAM is actually made unavailable by this approach.

 	(2) All other tiles are arranged to permit userspace access to the rest
             of the SDRAM.

      Barring point (1), there is nothing to protect kernel data against
      userspace damage - but this is uClinux.


  (*) Exceptions and Fixups

      Since the FR40x and FR55x CPUs that do not have full MMUs generate
      imprecise data error exceptions, there are currently no automatic fixup
      services available in uClinux. This includes misaligned memory access
      fixups.

      Userspace EFAULT errors can be trapped by issuing a MEMBAR instruction and
      forcing the fault to happen there.

      On the FR451, however, data exceptions are mostly precise, and so
      exception fixup handling is implemented as normal.


  (*) Userspace Breakpoints

      The ptrace() system call supports the following userspace debugging
      features:

 	(1) Hardware assisted single step.

 	(2) Breakpoint via the FR-V "BREAK" instruction.

 	(3) Breakpoint via the FR-V "TIRA GR0, #1" instruction.

 	(4) Syscall entry/exit trap.

      Each of the above generates a SIGTRAP.


  (*) On-Chip Serial Ports

      The FR-V on-chip serial ports are made available as ttyS0 and ttyS1. Note
      that if the GDB stub is compiled in, ttyS1 will not actually be available
      as it will be being used for the GDB stub.

      These ports can be made by:

 	mknod /dev/ttyS0 c 4 64
 	mknod /dev/ttyS1 c 4 65


  (*) Maskable Interrupts

      Level 15 (Non-maskable) interrupts are dealt with by the GDB stub if
      present, and cause a panic if not. If the GDB stub is present, ttyS1's
      interrupts are rated at level 15.

      All other interrupts are distributed over the set of available priorities
      so that no IRQs are shared where possible. The arch interrupt handling
      routines attempt to disentangle the various sources available through the
      CPU's own multiplexor, and those on off-CPU peripherals.


  (*) Accessing PCI Devices

      Where PCI is available, care must be taken when dealing with drivers that
      access PCI devices. PCI devices present their data in little-endian form,
      but the CPU sees it in big-endian form. The macros in asm/io.h try to get
      this right, but may not under all circumstances...


  (*) Ax88796 Ethernet Driver

      The MB93093 PDK board has an Ax88796 ethernet chipset (an NE2000 clone). A
      driver has been written to deal specifically with this. The driver
      provides MII services for the card.

      The driver can be configured by running make xconfig, and going to:

 	(*) Network device support
 	    - turn on "Network device support"
 	    (*) Ethernet (10 or 100Mbit)
 		- turn on "Ethernet (10 or 100Mbit)"
 		- turn on "AX88796 NE2000 compatible chipset"

      The driver can be found in:

 	drivers/net/ax88796.c
 	include/asm/ax88796.h


  (*) WorkRAM Driver

      This driver provides a character device that permits access to the WorkRAM
      that can be found on the FR451 CPU. Each page is accessible through a
      separate minor number, thereby permitting each page to have its own
      filesystem permissions set on the device file.

      The device files should be:

 	mknod /dev/frv/workram0 c 240 0
 	mknod /dev/frv/workram1 c 240 1
 	mknod /dev/frv/workram2 c 240 2
 	...

      The driver will not permit the opening of any device file that does not
      correspond to at least a partial page of WorkRAM. So the first device file
      is the only one available on the FR451. If any other CPU is detected, none
      of the devices will be openable.

      The devices can be accessed with read, write and llseek, and can also be
      mmapped. If they're mmapped, they will only map at the appropriate
      0x7e8nnnnn address on linux and at the 0xfe8nnnnn address on uClinux. If
      MAP_FIXED is not specified, the appropriate address will be chosen anyway.

      The mappings must be MAP_SHARED not MAP_PRIVATE, and must not be
      PROT_EXEC. They must also start at file offset 0, and must not be longer
      than one page in size.

      This driver can be configured by running make xconfig, and going to:

 	(*) Character devices
 	    - turn on "Fujitsu FR-V CPU WorkRAM support"


  (*) Dynamic data cache write mode changing

      It is possible to view and to change the data cache's write mode through
      the /proc/sys/frv/cache-mode file while the kernel is running. There are
      two modes available:

 	NAME	MEANING
 	=====	==========================================
 	wthru	Data cache is in Write-Through mode
 	wback	Data cache is in Write-Back/Copy-Back mode

      To read the cache mode:

 	# cat /proc/sys/frv/cache-mode
 	wthru

      To change the cache mode:

 	# echo wback >/proc/sys/frv/cache-mode
 	# cat /proc/sys/frv/cache-mode
 	wback


  (*) MMU Context IDs and Pinning

      On MMU Linux the CPU supports the concept of a context ID in its MMU to
      make it more efficient (TLB entries are labelled with a context ID to link
      them to specific tasks).

      Normally once a context ID is allocated, it will remain affixed to a task
      or CLONE_VM'd group of tasks for as long as it exists. However, since the
      kernel is capable of supporting more tasks than there are possible ID
      numbers, the kernel will pass context IDs from one task to another if
      there are insufficient available.

      The context ID currently in use by a task can be viewed in /proc:

 	# grep CXNR /proc/1/status
 	CXNR: 1

      Note that kernel threads do not have a userspace context, and so will not
      show a CXNR entry in that file.

      Under some circumstances, however, it is desirable to pin a context ID on
      a process such that the kernel won't pass it on. This can be done by
      writing the process ID of the target process to a special file:

 	# echo 17 >/proc/sys/frv/pin-cxnr

      Reading from the file will then show the context ID pinned.

 	# cat /proc/sys/frv/pin-cxnr
 	4

      The context ID will remain pinned as long as any process is using that
      context, i.e.: when the all the subscribing processes have exited or
      exec'd; or when an unpinning request happens:

 	# echo 0 >/proc/sys/frv/pin-cxnr

      When there isn't a pinned context, the file shows -1:

 	# cat /proc/sys/frv/pin-cxnr
 	-1
	===========================
	FUJITSU FR-V LINUX FEATURES
	===========================

	This kernel port has a number of features of which the user should be aware:

	(*) Linux and uClinux

	The FR-V architecture port supports both normal MMU linux and uClinux out
	of the same sources.


	(*) CPU support

	Support for the FR401, FR403, FR405, FR451 and FR555 CPUs should work with
	the same uClinux kernel configuration.

	In normal (MMU) Linux mode, only the FR451 CPU will work as that is the
	only one with a suitably featured CPU.

	The kernel is written and compiled with the assumption that only the
	bottom 32 GR registers and no FR registers will be used by the kernel
	itself, however all extra userspace registers will be saved on context
	switch. Note that since most CPUs can't support lazy switching, no attempt
	is made to do lazy register saving where that would be possible (FR555
	only currently).


	(*) Board support

	The board on which the kernel will run can be configured on the "Processor
	type and features" configuration tab.

	Set the System to "MB93093-PDK" to boot from the MB93093 (FR403) PDK.

	Set the System to "MB93091-VDK" to boot from the CB11, CB30, CB41, CB60,
	CB70 or CB451 VDK boards. Set the Motherboard setting to "MB93090-MB00" to
	boot with the standard ATA90590B VDK motherboard, and set it to "None" to
	boot without any motherboard.


	(*) Binary Formats

	The only userspace binary format supported is FDPIC ELF. Normal ELF, FLAT
	and AOUT binaries are not supported for this architecture.

	FDPIC ELF supports shared library and program interpreter facilities.


	(*) Scheduler Speed

	The kernel scheduler runs at 100Hz irrespective of the clock speed on this
	architecture. This value is set in asm/param.h (see the HZ macro defined
	there).


	(*) Normal (MMU) Linux Memory Layout.

	See mmu-layout.txt in this directory for a description of the normal linux
	memory layout

	See include/asm-frv/mem-layout.h for constants pertaining to the memory
	layout.

	See include/asm-frv/mb-regs.h for the constants pertaining to the I/O bus
	controller configuration.


	(*) uClinux Memory Layout

	The memory layout used by the uClinux kernel is as follows:

	0x00000000 - 0x00000FFF Null pointer catch page
	0x20000000 - 0x200FFFFF CS2# [PDK] FPGA
	0xC0000000 - 0xCFFFFFFF SDRAM
	0xC0000000 Base of Linux kernel image
	0xE0000000 - 0xEFFFFFFF CS2# [VDK] SLBUS/PCI window
	0xF0000000 - 0xF0FFFFFF CS5# MB93493 CSC area (DAV daughter board)
	0xF1000000 - 0xF1FFFFFF CS7# [CB70/CB451] CPU-card PCMCIA port space
	0xFC000000 - 0xFC0FFFFF CS1# [VDK] MB86943 config space
	0xFC100000 - 0xFC1FFFFF CS6# [CB70/CB451] CPU-card DM9000 NIC space
	0xFC100000 - 0xFC1FFFFF CS6# [PDK] AX88796 NIC space
	0xFC200000 - 0xFC2FFFFF CS3# MB93493 CSR area (DAV daughter board)
	0xFD000000 - 0xFDFFFFFF CS4# [CB70/CB451] CPU-card extra flash space
	0xFE000000 - 0xFEFFFFFF Internal CPU peripherals
	0xFF000000 - 0xFF1FFFFF CS0# Flash 1
	0xFF200000 - 0xFF3FFFFF CS0# Flash 2
	0xFFC00000 - 0xFFC0001F CS0# [VDK] FPGA

	The kernel reads the size of the SDRAM from the memory bus controller
	registers by default.

	The kernel initialisation code (1) adjusts the SDRAM base addresses to
	move the SDRAM to desired address, (2) moves the kernel image down to the
	bottom of SDRAM, (3) adjusts the bus controller registers to move I/O
	windows, and (4) rearranges the protection registers to protect all of
	this.

	The reasons for doing this are: (1) the page at address 0 should be
	inaccessible so that NULL pointer errors can be caught; and (2) the bottom
	three quarters are left unoccupied so that an FR-V CPU with an MMU can use
	it for virtual userspace mappings.

	See include/asm-frv/mem-layout.h for constants pertaining to the memory
	layout.

	See include/asm-frv/mb-regs.h for the constants pertaining to the I/O bus
	controller configuration.


	(*) uClinux Memory Protection

	A DAMPR register is used to cover the entire region used for I/O
	(0xE0000000 - 0xFFFFFFFF). This permits the kernel to make uncached
	accesses to this region. Userspace is not permitted to access it.

	The DAMPR/IAMPR protection registers not in use for any other purpose are
	tiled over the top of the SDRAM such that:

	(1) The core kernel image is covered by as small a tile as possible
	granting only the kernel access to the underlying data, whilst
	making sure no SDRAM is actually made unavailable by this approach.

	(2) All other tiles are arranged to permit userspace access to the rest
	of the SDRAM.

	Barring point (1), there is nothing to protect kernel data against
	userspace damage - but this is uClinux.


	(*) Exceptions and Fixups

	Since the FR40x and FR55x CPUs that do not have full MMUs generate
	imprecise data error exceptions, there are currently no automatic fixup
	services available in uClinux. This includes misaligned memory access
	fixups.

	Userspace EFAULT errors can be trapped by issuing a MEMBAR instruction and
	forcing the fault to happen there.

	On the FR451, however, data exceptions are mostly precise, and so
	exception fixup handling is implemented as normal.


	(*) Userspace Breakpoints

	The ptrace() system call supports the following userspace debugging
	features:

	(1) Hardware assisted single step.

	(2) Breakpoint via the FR-V "BREAK" instruction.

	(3) Breakpoint via the FR-V "TIRA GR0, #1" instruction.

	(4) Syscall entry/exit trap.

	Each of the above generates a SIGTRAP.


	(*) On-Chip Serial Ports

	The FR-V on-chip serial ports are made available as ttyS0 and ttyS1. Note
	that if the GDB stub is compiled in, ttyS1 will not actually be available
	as it will be being used for the GDB stub.

	These ports can be made by:

	mknod /dev/ttyS0 c 4 64
	mknod /dev/ttyS1 c 4 65


	(*) Maskable Interrupts

	Level 15 (Non-maskable) interrupts are dealt with by the GDB stub if
	present, and cause a panic if not. If the GDB stub is present, ttyS1's
	interrupts are rated at level 15.

	All other interrupts are distributed over the set of available priorities
	so that no IRQs are shared where possible. The arch interrupt handling
	routines attempt to disentangle the various sources available through the
	CPU's own multiplexor, and those on off-CPU peripherals.


	(*) Accessing PCI Devices

	Where PCI is available, care must be taken when dealing with drivers that
	access PCI devices. PCI devices present their data in little-endian form,
	but the CPU sees it in big-endian form. The macros in asm/io.h try to get
	this right, but may not under all circumstances...


	(*) Ax88796 Ethernet Driver

	The MB93093 PDK board has an Ax88796 ethernet chipset (an NE2000 clone). A
	driver has been written to deal specifically with this. The driver
	provides MII services for the card.

	The driver can be configured by running make xconfig, and going to:

	(*) Network device support
	- turn on "Network device support"
	(*) Ethernet (10 or 100Mbit)
	- turn on "Ethernet (10 or 100Mbit)"
	- turn on "AX88796 NE2000 compatible chipset"

	The driver can be found in:

	drivers/net/ax88796.c
	include/asm/ax88796.h


	(*) WorkRAM Driver

	This driver provides a character device that permits access to the WorkRAM
	that can be found on the FR451 CPU. Each page is accessible through a
	separate minor number, thereby permitting each page to have its own
	filesystem permissions set on the device file.

	The device files should be:

	mknod /dev/frv/workram0 c 240 0
	mknod /dev/frv/workram1 c 240 1
	mknod /dev/frv/workram2 c 240 2
	...

	The driver will not permit the opening of any device file that does not
	correspond to at least a partial page of WorkRAM. So the first device file
	is the only one available on the FR451. If any other CPU is detected, none
	of the devices will be openable.

	The devices can be accessed with read, write and llseek, and can also be
	mmapped. If they're mmapped, they will only map at the appropriate
	0x7e8nnnnn address on linux and at the 0xfe8nnnnn address on uClinux. If
	MAP_FIXED is not specified, the appropriate address will be chosen anyway.

	The mappings must be MAP_SHARED not MAP_PRIVATE, and must not be
	PROT_EXEC. They must also start at file offset 0, and must not be longer
	than one page in size.

	This driver can be configured by running make xconfig, and going to:

	(*) Character devices
	- turn on "Fujitsu FR-V CPU WorkRAM support"


	(*) Dynamic data cache write mode changing

	It is possible to view and to change the data cache's write mode through
	the /proc/sys/frv/cache-mode file while the kernel is running. There are
	two modes available:

	NAME MEANING
	===== ==========================================
	wthru Data cache is in Write-Through mode
	wback Data cache is in Write-Back/Copy-Back mode

	To read the cache mode:

	# cat /proc/sys/frv/cache-mode
	wthru

	To change the cache mode:

	# echo wback >/proc/sys/frv/cache-mode
	# cat /proc/sys/frv/cache-mode
	wback


	(*) MMU Context IDs and Pinning

	On MMU Linux the CPU supports the concept of a context ID in its MMU to
	make it more efficient (TLB entries are labelled with a context ID to link
	them to specific tasks).

	Normally once a context ID is allocated, it will remain affixed to a task
	or CLONE_VM'd group of tasks for as long as it exists. However, since the
	kernel is capable of supporting more tasks than there are possible ID
	numbers, the kernel will pass context IDs from one task to another if
	there are insufficient available.

	The context ID currently in use by a task can be viewed in /proc:

	# grep CXNR /proc/1/status
	CXNR: 1

	Note that kernel threads do not have a userspace context, and so will not
	show a CXNR entry in that file.

	Under some circumstances, however, it is desirable to pin a context ID on
	a process such that the kernel won't pass it on. This can be done by
	writing the process ID of the target process to a special file:

	# echo 17 >/proc/sys/frv/pin-cxnr

	Reading from the file will then show the context ID pinned.

	# cat /proc/sys/frv/pin-cxnr
	4

	The context ID will remain pinned as long as any process is using that
	context, i.e.: when the all the subscribing processes have exited or
	exec'd; or when an unpinning request happens:

	# echo 0 >/proc/sys/frv/pin-cxnr

	When there isn't a pinned context, the file shows -1:

	# cat /proc/sys/frv/pin-cxnr
	-1