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+
+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 an userspace application, it is not the role of the 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 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. Device 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's also a in-kernel API of the arbiter in the case of DRM, vgacon and
+others which may use the arbiter.
+
+
+I.2 libpciaccess
+----------------
+
+To use the vga arbiter char device it was implemented an API inside the
+libpciaccess library. One fieldd 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 usually need to keep informed how many cards are being
+arbitrated, so for instance if there's only one then it can totally escape the
+scheme.
+
+
+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