Documentation/vgaarbiter.txt - kernel/common - Git at Google


 VGA Arbiter
 ===========

 Graphic devices are accessed through ranges in I/O or memory space. While most
 modern devices allow relocation of such ranges, some "Legacy" VGA devices
 implemented on PCI will typically have the same "hard-decoded" addresses as
 they did on ISA. For more details see "PCI Bus Binding to IEEE Std 1275-1994
 Standard for Boot (Initialization Configuration) Firmware Revision 2.1"
 Section 7, Legacy Devices.

 The Resource Access Control (RAC) module inside the X server [0] existed for
 the legacy VGA arbitration task (besides other bus management tasks) when more
 than one legacy device co-exists on the same machine. But the problem happens
 when these devices are trying to be accessed by different userspace clients
 (e.g. two server in parallel). Their address assignments conflict. Moreover,
 ideally, being a userspace application, it is not the role of the X server to
 control bus resources. Therefore an arbitration scheme outside of the X server
 is needed to control the sharing of these resources. This document introduces
 the operation of the VGA arbiter implemented for the Linux kernel.

 ----------------------------------------------------------------------------

 I.  Details and Theory of Operation
         I.1 vgaarb
         I.2 libpciaccess
         I.3 xf86VGAArbiter (X server implementation)
 II. Credits
 III.References


 I. Details and Theory of Operation
 ==================================

 I.1 vgaarb
 ----------

 The vgaarb is a module of the Linux Kernel. When it is initially loaded, it
 scans all PCI devices and adds the VGA ones inside the arbitration. The
 arbiter then enables/disables the decoding on different devices of the VGA
 legacy instructions. Devices which do not want/need to use the arbiter may
 explicitly tell it by calling vga_set_legacy_decoding().

 The kernel exports a char device interface (/dev/vga_arbiter) to the clients,
 which has the following semantics:

  open       : open user instance of the arbiter. By default, it's attached to
               the default VGA device of the system.

  close      : close user instance. Release locks made by the user

  read       : return a string indicating the status of the target like:

               "<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)"

               An IO state string is of the form {io,mem,io+mem,none}, mc and
               ic are respectively mem and io lock counts (for debugging/
               diagnostic only). "decodes" indicate what the card currently
               decodes, "owns" indicates what is currently enabled on it, and
               "locks" indicates what is locked by this card. If the card is
               unplugged, we get "invalid" then for card_ID and an -ENODEV
               error is returned for any command until a new card is targeted.


  write       : write a command to the arbiter. List of commands:

   target <card_ID>   : switch target to card <card_ID> (see below)
   lock <io_state>    : acquires locks on target ("none" is an invalid io_state)
   trylock <io_state> : non-blocking acquire locks on target (returns EBUSY if
                        unsuccessful)
   unlock <io_state>  : release locks on target
   unlock all         : release all locks on target held by this user (not
                        implemented yet)
   decodes <io_state> : set the legacy decoding attributes for the card

   poll               : event if something changes on any card (not just the
                        target)

   card_ID is of the form "PCI:domain:bus:dev.fn". It can be set to "default"
   to go back to the system default card (TODO: not implemented yet). Currently,
   only PCI is supported as a prefix, but the userland API may support other bus
   types in the future, even if the current kernel implementation doesn't.

 Note about locks:

 The driver keeps track of which user has which locks on which card. It
 supports stacking, like the kernel one. This complexifies the implementation
 a bit, but makes the arbiter more tolerant to user space problems and able
 to properly cleanup in all cases when a process dies.
 Currently, a max of 16 cards can have locks simultaneously issued from
 user space for a given user (file descriptor instance) of the arbiter.

 In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to
 notify them being added/removed in the system and automatically added/removed
 in the arbiter.

 There is also an in-kernel API of the arbiter in case DRM, vgacon, or other
 drivers want to use it.


 I.2 libpciaccess
 ----------------

 To use the vga arbiter char device it was implemented an API inside the
 libpciaccess library. One field was added to struct pci_device (each device
 on the system):

     /* the type of resource decoded by the device */
     int vgaarb_rsrc;

 Besides it, in pci_system were added:

     int vgaarb_fd;
     int vga_count;
     struct pci_device *vga_target;
     struct pci_device *vga_default_dev;


 The vga_count is used to track how many cards are being arbitrated, so for
 instance, if there is only one card, then it can completely escape arbitration.


 These functions below acquire VGA resources for the given card and mark those
 resources as locked. If the resources requested are "normal" (and not legacy)
 resources, the arbiter will first check whether the card is doing legacy
 decoding for that type of resource. If yes, the lock is "converted" into a
 legacy resource lock. The arbiter will first look for all VGA cards that
 might conflict and disable their IOs and/or Memory access, including VGA
 forwarding on P2P bridges if necessary, so that the requested resources can
 be used. Then, the card is marked as locking these resources and the IO and/or
 Memory access is enabled on the card (including VGA forwarding on parent
 P2P bridges if any). In the case of vga_arb_lock(), the function will block
 if some conflicting card is already locking one of the required resources (or
 any resource on a different bus segment, since P2P bridges don't differentiate
 VGA memory and IO afaik). If the card already owns the resources, the function
 succeeds.  vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested
 calls are supported (a per-resource counter is maintained).


 Set the target device of this client.
     int  pci_device_vgaarb_set_target   (struct pci_device *dev);


 For instance, in x86 if two devices on the same bus want to lock different
 resources, both will succeed (lock). If devices are in different buses and
 trying to lock different resources, only the first who tried succeeds.
     int  pci_device_vgaarb_lock         (void);
     int  pci_device_vgaarb_trylock      (void);

 Unlock resources of device.
     int  pci_device_vgaarb_unlock       (void);

 Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA
 Memory, both, or none. All cards default to both, the card driver (fbdev for
 example) should tell the arbiter if it has disabled legacy decoding, so the
 card can be left out of the arbitration process (and can be safe to take
 interrupts at any time.
     int  pci_device_vgaarb_decodes      (int new_vgaarb_rsrc);

 Connects to the arbiter device, allocates the struct
     int  pci_device_vgaarb_init         (void);

 Close the connection
     void pci_device_vgaarb_fini         (void);


 I.3 xf86VGAArbiter (X server implementation)
 --------------------------------------------

 (TODO)

 X server basically wraps all the functions that touch VGA registers somehow.


 II. Credits
 ===========

 Benjamin Herrenschmidt (IBM?) started this work when he discussed such design
 with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and
 Tiago Vignatti (both of C3SL/Federal University of Paraná) proceeded his work
 enhancing the kernel code to adapt as a kernel module and also did the
 implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave
 Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree.


 III. References
 ==============

 [0] http://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347
 [1] http://lists.freedesktop.org/archives/xorg/2005-March/006663.html
 [2] http://lists.freedesktop.org/archives/xorg/2005-March/006745.html
 [3] http://lists.freedesktop.org/archives/xorg/2007-October/029507.html

	VGA Arbiter
	===========

	Graphic devices are accessed through ranges in I/O or memory space. While most
	modern devices allow relocation of such ranges, some "Legacy" VGA devices
	implemented on PCI will typically have the same "hard-decoded" addresses as
	they did on ISA. For more details see "PCI Bus Binding to IEEE Std 1275-1994
	Standard for Boot (Initialization Configuration) Firmware Revision 2.1"
	Section 7, Legacy Devices.

	The Resource Access Control (RAC) module inside the X server [0] existed for
	the legacy VGA arbitration task (besides other bus management tasks) when more
	than one legacy device co-exists on the same machine. But the problem happens
	when these devices are trying to be accessed by different userspace clients
	(e.g. two server in parallel). Their address assignments conflict. Moreover,
	ideally, being a userspace application, it is not the role of the X server to
	control bus resources. Therefore an arbitration scheme outside of the X server
	is needed to control the sharing of these resources. This document introduces
	the operation of the VGA arbiter implemented for the Linux kernel.

	----------------------------------------------------------------------------

	I. Details and Theory of Operation
	I.1 vgaarb
	I.2 libpciaccess
	I.3 xf86VGAArbiter (X server implementation)
	II. Credits
	III.References


	I. Details and Theory of Operation
	==================================

	I.1 vgaarb
	----------

	The vgaarb is a module of the Linux Kernel. When it is initially loaded, it
	scans all PCI devices and adds the VGA ones inside the arbitration. The
	arbiter then enables/disables the decoding on different devices of the VGA
	legacy instructions. Devices which do not want/need to use the arbiter may
	explicitly tell it by calling vga_set_legacy_decoding().

	The kernel exports a char device interface (/dev/vga_arbiter) to the clients,
	which has the following semantics:

	open : open user instance of the arbiter. By default, it's attached to
	the default VGA device of the system.

	close : close user instance. Release locks made by the user

	read : return a string indicating the status of the target like:

	"<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)"

	An IO state string is of the form {io,mem,io+mem,none}, mc and
	ic are respectively mem and io lock counts (for debugging/
	diagnostic only). "decodes" indicate what the card currently
	decodes, "owns" indicates what is currently enabled on it, and
	"locks" indicates what is locked by this card. If the card is
	unplugged, we get "invalid" then for card_ID and an -ENODEV
	error is returned for any command until a new card is targeted.


	write : write a command to the arbiter. List of commands:

	target <card_ID> : switch target to card <card_ID> (see below)
	lock <io_state> : acquires locks on target ("none" is an invalid io_state)
	trylock <io_state> : non-blocking acquire locks on target (returns EBUSY if
	unsuccessful)
	unlock <io_state> : release locks on target
	unlock all : release all locks on target held by this user (not
	implemented yet)
	decodes <io_state> : set the legacy decoding attributes for the card

	poll : event if something changes on any card (not just the
	target)

	card_ID is of the form "PCI:domain:bus:dev.fn". It can be set to "default"
	to go back to the system default card (TODO: not implemented yet). Currently,
	only PCI is supported as a prefix, but the userland API may support other bus
	types in the future, even if the current kernel implementation doesn't.

	Note about locks:

	The driver keeps track of which user has which locks on which card. It
	supports stacking, like the kernel one. This complexifies the implementation
	a bit, but makes the arbiter more tolerant to user space problems and able
	to properly cleanup in all cases when a process dies.
	Currently, a max of 16 cards can have locks simultaneously issued from
	user space for a given user (file descriptor instance) of the arbiter.

	In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to
	notify them being added/removed in the system and automatically added/removed
	in the arbiter.

	There is also an in-kernel API of the arbiter in case DRM, vgacon, or other
	drivers want to use it.


	I.2 libpciaccess
	----------------

	To use the vga arbiter char device it was implemented an API inside the
	libpciaccess library. One field was added to struct pci_device (each device
	on the system):

	/* the type of resource decoded by the device */
	int vgaarb_rsrc;

	Besides it, in pci_system were added:

	int vgaarb_fd;
	int vga_count;
	struct pci_device *vga_target;
	struct pci_device *vga_default_dev;


	The vga_count is used to track how many cards are being arbitrated, so for
	instance, if there is only one card, then it can completely escape arbitration.


	These functions below acquire VGA resources for the given card and mark those
	resources as locked. If the resources requested are "normal" (and not legacy)
	resources, the arbiter will first check whether the card is doing legacy
	decoding for that type of resource. If yes, the lock is "converted" into a
	legacy resource lock. The arbiter will first look for all VGA cards that
	might conflict and disable their IOs and/or Memory access, including VGA
	forwarding on P2P bridges if necessary, so that the requested resources can
	be used. Then, the card is marked as locking these resources and the IO and/or
	Memory access is enabled on the card (including VGA forwarding on parent
	P2P bridges if any). In the case of vga_arb_lock(), the function will block
	if some conflicting card is already locking one of the required resources (or
	any resource on a different bus segment, since P2P bridges don't differentiate
	VGA memory and IO afaik). If the card already owns the resources, the function
	succeeds. vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested
	calls are supported (a per-resource counter is maintained).


	Set the target device of this client.
	int pci_device_vgaarb_set_target (struct pci_device *dev);


	For instance, in x86 if two devices on the same bus want to lock different
	resources, both will succeed (lock). If devices are in different buses and
	trying to lock different resources, only the first who tried succeeds.
	int pci_device_vgaarb_lock (void);
	int pci_device_vgaarb_trylock (void);

	Unlock resources of device.
	int pci_device_vgaarb_unlock (void);

	Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA
	Memory, both, or none. All cards default to both, the card driver (fbdev for
	example) should tell the arbiter if it has disabled legacy decoding, so the
	card can be left out of the arbitration process (and can be safe to take
	interrupts at any time.
	int pci_device_vgaarb_decodes (int new_vgaarb_rsrc);

	Connects to the arbiter device, allocates the struct
	int pci_device_vgaarb_init (void);

	Close the connection
	void pci_device_vgaarb_fini (void);


	I.3 xf86VGAArbiter (X server implementation)
	--------------------------------------------

	(TODO)

	X server basically wraps all the functions that touch VGA registers somehow.


	II. Credits
	===========

	Benjamin Herrenschmidt (IBM?) started this work when he discussed such design
	with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and
	Tiago Vignatti (both of C3SL/Federal University of Paraná) proceeded his work
	enhancing the kernel code to adapt as a kernel module and also did the
	implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave
	Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree.


	III. References
	==============

	[0] http://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347
	[1] http://lists.freedesktop.org/archives/xorg/2005-March/006663.html
	[2] http://lists.freedesktop.org/archives/xorg/2005-March/006745.html
	[3] http://lists.freedesktop.org/archives/xorg/2007-October/029507.html