Commit b754b35b authored by Daniel Vetter's avatar Daniel Vetter
Browse files

vgaarbiter: rst-ifiy and polish kerneldoc 



Move the documentation into Documentation/gpu, link it up and pull in
the kernel doc.

No actual text changes except that I did polish the kerneldoc a bit,
especially for vga_client_register().

v2: Remove some rst from vga-switcheroo.rst that I don't understand,
but which seems to be the reason why the new vgaarbiter.rst sometimes
drops out of the sidebar index.

v3: Drop one level of headings and clarify the vgaarb one a bit.

v4: Fix some typos (Sean).

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: default avatarSean Paul <seanpaul@chromium.org>
Signed-off-by: default avatarDaniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-20-git-send-email-daniel.vetter@ffwll.ch
parent b3c6c8bf

Documentation/gpu/index.rst

+1 −0
Original line number Diff line number Diff line
@@ -12,3 +12,4 @@ Linux GPU Driver Developer's Guide 
   drm-uapi

   i915

   vga-switcheroo

   vgaarbiter

Documentation/gpu/vga-switcheroo.rst

+0 −2
Original line number Diff line number Diff line 
.. _vga_switcheroo:



==============

VGA Switcheroo

==============

Documentation/vgaarbiter.txt→Documentation/gpu/vgaarbiter.rst

+80 −81
Original line number Diff line number Diff line 


===========

VGA Arbiter

===========
@@ -19,21 +19,8 @@ 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

----------

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
@@ -44,12 +31,15 @@ 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.

open

        Opens a 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

close

        Close a user instance. Release locks made by the user



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

read

        Return a string indicating the status of the target like:



        "<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)"
@@ -62,19 +52,26 @@ which has the following semantics: 
        error is returned for any command until a new card is targeted.





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

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

        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

        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)

        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,
@@ -97,29 +94,35 @@ 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:



I.2 libpciaccess

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

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):

on the system)::



    /* the type of resource decoded by the device */

    int vgaarb_rsrc;



Besides it, in pci_system were added:

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
@@ -136,44 +139,44 @@ 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. ::



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.

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.

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.

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);

Connects to the arbiter device, allocates the struct ::



Close the connection

    void pci_device_vgaarb_fini         (void);

    int  pci_device_vgaarb_init         (void);



Close the connection ::



I.3 xf86VGAArbiter (X server implementation)

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

    void pci_device_vgaarb_fini         (void);



(TODO)

xf86VGAArbiter (X server implementation)

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



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





II. Credits

===========

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
@@ -182,11 +185,7 @@ 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
 
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

drivers/gpu/vga/vgaarb.c

+108 −2
Original line number Diff line number Diff line
@@ -131,7 +131,24 @@ static struct vga_device *vgadev_find(struct pci_dev *pdev) 
	return NULL;

}



/* Returns the default VGA device (vgacon's babe) */

/**

 * vga_default_device - return the default VGA device, for vgacon

 *

 * This can be defined by the platform. The default implementation

 * is rather dumb and will probably only work properly on single

 * vga card setups and/or x86 platforms.

 *

 * If your VGA default device is not PCI, you'll have to return

 * NULL here. In this case, I assume it will not conflict with

 * any PCI card. If this is not true, I'll have to define two archs

 * hooks for enabling/disabling the VGA default device if that is

 * possible. This may be a problem with real _ISA_ VGA cards, in

 * addition to a PCI one. I don't know at this point how to deal

 * with that card. Can theirs IOs be disabled at all ? If not, then

 * I suppose it's a matter of having the proper arch hook telling

 * us about it, so we basically never allow anybody to succeed a

 * vga_get()...

 */

struct pci_dev *vga_default_device(void)

{

	return vga_default;
@@ -356,6 +373,40 @@ static void __vga_put(struct vga_device *vgadev, unsigned int rsrc) 
		wake_up_all(&vga_wait_queue);

}



/**

 * vga_get - acquire & locks VGA resources

 * @pdev: pci device of the VGA card or NULL for the system default

 * @rsrc: bit mask of resources to acquire and lock

 * @interruptible: blocking should be interruptible by signals ?

 *

 * This function acquires VGA resources for the given card and mark those

 * resources locked. If the resource 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 accesses are

 * enabled on the card (including VGA forwarding on parent P2P bridges if any).

 *

 * This 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). You can indicate

 * whether this blocking should be interruptible by a signal (for userland

 * interface) or not.

 *

 * Must not be called at interrupt time or in atomic context.  If the card

 * already owns the resources, the function succeeds.  Nested calls are

 * supported (a per-resource counter is maintained)

 *

 * On success, release the VGA resource again with vga_put().

 *

 * Returns:

 *

 * 0 on success, negative error code on failure.

 */

int vga_get(struct pci_dev *pdev, unsigned int rsrc, int interruptible)

{

	struct vga_device *vgadev, *conflict;
@@ -408,6 +459,21 @@ int vga_get(struct pci_dev *pdev, unsigned int rsrc, int interruptible) 
}

EXPORT_SYMBOL(vga_get);



/**

 * vga_tryget - try to acquire & lock legacy VGA resources

 * @pdev: pci devivce of VGA card or NULL for system default

 * @rsrc: bit mask of resources to acquire and lock

 *

 * This function performs the same operation as vga_get(), but will return an

 * error (-EBUSY) instead of blocking if the resources are already locked by

 * another card. It can be called in any context

 *

 * On success, release the VGA resource again with vga_put().

 *

 * Returns:

 *

 * 0 on success, negative error code on failure.

 */

int vga_tryget(struct pci_dev *pdev, unsigned int rsrc)

{

	struct vga_device *vgadev;
@@ -435,6 +501,16 @@ bail: 
}

EXPORT_SYMBOL(vga_tryget);



/**

 * vga_put - release lock on legacy VGA resources

 * @pdev: pci device of VGA card or NULL for system default

 * @rsrc: but mask of resource to release

 *

 * This fuction releases resources previously locked by vga_get() or

 * vga_tryget(). The resources aren't disabled right away, so that a subsequence

 * vga_get() on the same card will succeed immediately. Resources have a

 * counter, so locks are only released if the counter reaches 0.

 */

void vga_put(struct pci_dev *pdev, unsigned int rsrc)

{

	struct vga_device *vgadev;
@@ -716,7 +792,37 @@ void vga_set_legacy_decoding(struct pci_dev *pdev, unsigned int decodes) 
}

EXPORT_SYMBOL(vga_set_legacy_decoding);



/* call with NULL to unregister */

/**

 * vga_client_register - register or unregister a VGA arbitration client

 * @pdev: pci device of the VGA client

 * @cookie: client cookie to be used in callbacks

 * @irq_set_state: irq state change callback

 * @set_vga_decode: vga decode change callback

 *

 * Clients have two callback mechanisms they can use.

 *

 * @irq_set_state callback: If a client can't disable its GPUs VGA

 * resources, then we need to be able to ask it to turn off its irqs when we

 * turn off its mem and io decoding.

 *

 * @set_vga_decode callback: If a client can disable its GPU VGA resource, it

 * will get a callback from this to set the encode/decode state.

 *

 * Rationale: we cannot disable VGA decode resources unconditionally some single

 * GPU laptops seem to require ACPI or BIOS access to the VGA registers to

 * control things like backlights etc.  Hopefully newer multi-GPU laptops do

 * something saner, and desktops won't have any special ACPI for this. The

 * driver will get a callback when VGA arbitration is first used by userspace

 * since some older X servers have issues.

 *

 * This function does not check whether a client for @pdev has been registered

 * already.

 *

 * To unregister just call this function with @irq_set_state and @set_vga_decode

 * both set to NULL for the same @pdev as originally used to register them.

 *

 * Returns: 0 on success, -1 on failure

 */

int vga_client_register(struct pci_dev *pdev, void *cookie,

			void (*irq_set_state)(void *cookie, bool state),

			unsigned int (*set_vga_decode)(void *cookie,

include/linux/vgaarb.h

+16 −112
Original line number Diff line number Diff line
@@ -73,34 +73,6 @@ static inline void vga_set_legacy_decoding(struct pci_dev *pdev, 
					   unsigned int decodes) { };

#endif



/**

 *     vga_get         - acquire & locks VGA resources

 *

 *     @pdev: pci device of the VGA card or NULL for the system default

 *     @rsrc: bit mask of resources to acquire and lock

 *     @interruptible: blocking should be interruptible by signals ?

 *

 *     This function acquires VGA resources for the given

 *     card and mark those resources locked. If the resource 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 accesse are enabled on the card (including

 *     VGA forwarding on parent P2P bridges if any).

 *     This 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 differenciate VGA memory and IO

 *     afaik). You can indicate whether this blocking should be interruptible

 *     by a signal (for userland interface) or not.

 *     Must not be called at interrupt time or in atomic context.

 *     If the card already owns the resources, the function succeeds.

 *     Nested calls are supported (a per-resource counter is maintained)

 */



#if defined(CONFIG_VGA_ARB)

extern int vga_get(struct pci_dev *pdev, unsigned int rsrc, int interruptible);

#else
@@ -109,10 +81,13 @@ static inline int vga_get(struct pci_dev *pdev, unsigned int rsrc, int interrupt 


/**

 * vga_get_interruptible

 * @pdev: pci device of the VGA card or NULL for the system default

 * @rsrc: bit mask of resources to acquire and lock

 *

 *     Shortcut to vga_get

 * Shortcut to vga_get with interruptible set to true.

 *

 * On success, release the VGA resource again with vga_put().

 */



static inline int vga_get_interruptible(struct pci_dev *pdev,

					unsigned int rsrc)

{
@@ -120,47 +95,26 @@ static inline int vga_get_interruptible(struct pci_dev *pdev, 
}



/**

 *     vga_get_uninterruptible

 * vga_get_uninterruptible - shortcut to vga_get()

 * @pdev: pci device of the VGA card or NULL for the system default

 * @rsrc: bit mask of resources to acquire and lock

 *

 * Shortcut to vga_get with interruptible set to false.

 *

 *     Shortcut to vga_get

 * On success, release the VGA resource again with vga_put().

 */



static inline int vga_get_uninterruptible(struct pci_dev *pdev,

					  unsigned int rsrc)

{

       return vga_get(pdev, rsrc, 0);

}



/**

 *     vga_tryget      - try to acquire & lock legacy VGA resources

 *

 *     @pdev: pci devivce of VGA card or NULL for system default

 *     @rsrc: bit mask of resources to acquire and lock

 *

 *     This function performs the same operation as vga_get(), but

 *     will return an error (-EBUSY) instead of blocking if the resources

 *     are already locked by another card. It can be called in any context

 */



#if defined(CONFIG_VGA_ARB)

extern int vga_tryget(struct pci_dev *pdev, unsigned int rsrc);

#else

static inline int vga_tryget(struct pci_dev *pdev, unsigned int rsrc) { return 0; }

#endif



/**

 *     vga_put         - release lock on legacy VGA resources

 *

 *     @pdev: pci device of VGA card or NULL for system default

 *     @rsrc: but mask of resource to release

 *

 *     This function releases resources previously locked by vga_get()

 *     or vga_tryget(). The resources aren't disabled right away, so

 *     that a subsequence vga_get() on the same card will succeed

 *     immediately. Resources have a counter, so locks are only

 *     released if the counter reaches 0.

 */



#if defined(CONFIG_VGA_ARB)

extern void vga_put(struct pci_dev *pdev, unsigned int rsrc);

#else
@@ -168,25 +122,6 @@ extern void vga_put(struct pci_dev *pdev, unsigned int rsrc); 
#endif





/**

 *     vga_default_device

 *

 *     This can be defined by the platform. The default implementation

 *     is rather dumb and will probably only work properly on single

 *     vga card setups and/or x86 platforms.

 *

 *     If your VGA default device is not PCI, you'll have to return

 *     NULL here. In this case, I assume it will not conflict with

 *     any PCI card. If this is not true, I'll have to define two archs

 *     hooks for enabling/disabling the VGA default device if that is

 *     possible. This may be a problem with real _ISA_ VGA cards, in

 *     addition to a PCI one. I don't know at this point how to deal

 *     with that card. Can theirs IOs be disabled at all ? If not, then

 *     I suppose it's a matter of having the proper arch hook telling

 *     us about it, so we basically never allow anybody to succeed a

 *     vga_get()...

 */



#ifdef CONFIG_VGA_ARB

extern struct pci_dev *vga_default_device(void);

extern void vga_set_default_device(struct pci_dev *pdev);
@@ -195,14 +130,11 @@ static inline struct pci_dev *vga_default_device(void) { return NULL; }; 
static inline void vga_set_default_device(struct pci_dev *pdev) { };

#endif



/**

 *     vga_conflicts

 *

/*

 * Architectures should define this if they have several

 * independent PCI domains that can afford concurrent VGA

 * decoding

 */



#ifndef __ARCH_HAS_VGA_CONFLICT

static inline int vga_conflicts(struct pci_dev *p1, struct pci_dev *p2)

{
@@ -210,34 +142,6 @@ static inline int vga_conflicts(struct pci_dev *p1, struct pci_dev *p2) 
}

#endif



/**

 *	vga_client_register

 *

 *	@pdev: pci device of the VGA client

 *	@cookie: client cookie to be used in callbacks

 *	@irq_set_state: irq state change callback

 *	@set_vga_decode: vga decode change callback

 *

 * 	return value: 0 on success, -1 on failure

 * 	Register a client with the VGA arbitration logic

 *

 *	Clients have two callback mechanisms they can use.

 *	irq enable/disable callback -

 *		If a client can't disable its GPUs VGA resources, then we

 *		need to be able to ask it to turn off its irqs when we

 *		turn off its mem and io decoding.

 *	set_vga_decode

 *		If a client can disable its GPU VGA resource, it will

 *		get a callback from this to set the encode/decode state

 *

 * Rationale: we cannot disable VGA decode resources unconditionally

 * some single GPU laptops seem to require ACPI or BIOS access to the

 * VGA registers to control things like backlights etc.

 * Hopefully newer multi-GPU laptops do something saner, and desktops

 * won't have any special ACPI for this.

 * They driver will get a callback when VGA arbitration is first used

 * by userspace since we some older X servers have issues.

 */

#if defined(CONFIG_VGA_ARB)

int vga_client_register(struct pci_dev *pdev, void *cookie,

			void (*irq_set_state)(void *cookie, bool state),

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