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: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-20-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/vga/vgaarb.c

@@ -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,