Documentation/gpu/vgaarbiter.rst - linux - 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.


            vgaarb kernel/userspace ABI

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


            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

            
                    Opens a user instance of the arbiter. By default, it's attached to the

            
                    default VGA device of the system.


            close

            
                    Close a 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.


            In-kernel interface

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


            .. kernel-doc:: include/linux/vgaarb.h

            
               :internal:


            .. kernel-doc:: drivers/gpu/vga/vgaarb.c

            
               :export:


            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);


            xf86VGAArbiter (X server implementation)

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


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


            References

            
            ----------


            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.


            0) https://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347

            
            1) https://lists.freedesktop.org/archives/xorg/2005-March/006663.html

            
            2) https://lists.freedesktop.org/archives/xorg/2005-March/006745.html

            
            3) https://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.

	vgaarb kernel/userspace ABI
	---------------------------

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

	close
	Close a 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.

	In-kernel interface
	-------------------

	.. kernel-doc:: include/linux/vgaarb.h
	:internal:

	.. kernel-doc:: drivers/gpu/vga/vgaarb.c
	:export:

	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);

	xf86VGAArbiter (X server implementation)
	----------------------------------------

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

	References
	----------

	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.

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