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VGA Arbiter
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===========
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Graphic devices are accessed through ranges in I/O or memory space. While most
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modern devices allow relocation of such ranges, some "Legacy" VGA devices
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implemented on PCI will typically have the same "hard-decoded" addresses as
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they did on ISA. For more details see "PCI Bus Binding to IEEE Std 1275-1994
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Standard for Boot (Initialization Configuration) Firmware Revision 2.1"
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Section 7, Legacy Devices.
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The Resource Access Control (RAC) module inside the X server [0] existed for
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the legacy VGA arbitration task (besides other bus management tasks) when more
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than one legacy device co-exists on the same machine. But the problem happens
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when these devices are trying to be accessed by different userspace clients
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(e.g. two server in parallel). Their address assignments conflict. Moreover,
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ideally, being an userspace application, it is not the role of the the X
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server to control bus resources. Therefore an arbitration scheme outside of
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the X server is needed to control the sharing of these resources. This
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document introduces the operation of the VGA arbiter implemented for Linux
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kernel.
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----------------------------------------------------------------------------
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I. Details and Theory of Operation
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I.1 vgaarb
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I.2 libpciaccess
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I.3 xf86VGAArbiter (X server implementation)
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II. Credits
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III.References
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I. Details and Theory of Operation
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==================================
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I.1 vgaarb
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----------
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The vgaarb is a module of the Linux Kernel. When it is initially loaded, it
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scans all PCI devices and adds the VGA ones inside the arbitration. The
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arbiter then enables/disables the decoding on different devices of the VGA
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legacy instructions. Device which do not want/need to use the arbiter may
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explicitly tell it by calling vga_set_legacy_decoding().
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The kernel exports a char device interface (/dev/vga_arbiter) to the clients,
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which has the following semantics:
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open : open user instance of the arbiter. By default, it's attached to
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the default VGA device of the system.
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close : close user instance. Release locks made by the user
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read : return a string indicating the status of the target like:
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"<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)"
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An IO state string is of the form {io,mem,io+mem,none}, mc and
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ic are respectively mem and io lock counts (for debugging/
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diagnostic only). "decodes" indicate what the card currently
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decodes, "owns" indicates what is currently enabled on it, and
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"locks" indicates what is locked by this card. If the card is
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unplugged, we get "invalid" then for card_ID and an -ENODEV
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error is returned for any command until a new card is targeted.
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write : write a command to the arbiter. List of commands:
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target <card_ID> : switch target to card <card_ID> (see below)
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lock <io_state> : acquires locks on target ("none" is an invalid io_state)
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trylock <io_state> : non-blocking acquire locks on target (returns EBUSY if
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unsuccessful)
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unlock <io_state> : release locks on target
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unlock all : release all locks on target held by this user (not
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implemented yet)
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decodes <io_state> : set the legacy decoding attributes for the card
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poll : event if something changes on any card (not just the
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target)
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card_ID is of the form "PCI:domain:bus:dev.fn". It can be set to "default"
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to go back to the system default card (TODO: not implemented yet). Currently,
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only PCI is supported as a prefix, but the userland API may support other bus
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types in the future, even if the current kernel implementation doesn't.
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Note about locks:
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The driver keeps track of which user has which locks on which card. It
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supports stacking, like the kernel one. This complexifies the implementation
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a bit, but makes the arbiter more tolerant to user space problems and able
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to properly cleanup in all cases when a process dies.
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Currently, a max of 16 cards can have locks simultaneously issued from
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user space for a given user (file descriptor instance) of the arbiter.
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In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to
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notify them being added/removed in the system and automatically added/removed
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in the arbiter.
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There's also a in-kernel API of the arbiter in the case of DRM, vgacon and
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others which may use the arbiter.
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I.2 libpciaccess
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----------------
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To use the vga arbiter char device it was implemented an API inside the
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libpciaccess library. One field was added to struct pci_device (each device
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on the system):
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/* the type of resource decoded by the device */
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int vgaarb_rsrc;
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Besides it, in pci_system were added:
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int vgaarb_fd;
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int vga_count;
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struct pci_device *vga_target;
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struct pci_device *vga_default_dev;
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The vga_count is usually need to keep informed how many cards are being
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arbitrated, so for instance if there's only one then it can totally escape the
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scheme.
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These functions below acquire VGA resources for the given card and mark those
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resources as locked. If the resources requested are "normal" (and not legacy)
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resources, the arbiter will first check whether the card is doing legacy
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decoding for that type of resource. If yes, the lock is "converted" into a
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legacy resource lock. The arbiter will first look for all VGA cards that
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might conflict and disable their IOs and/or Memory access, including VGA
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forwarding on P2P bridges if necessary, so that the requested resources can
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be used. Then, the card is marked as locking these resources and the IO and/or
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Memory access is enabled on the card (including VGA forwarding on parent
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P2P bridges if any). In the case of vga_arb_lock(), the function will block
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if some conflicting card is already locking one of the required resources (or
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any resource on a different bus segment, since P2P bridges don't differentiate
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VGA memory and IO afaik). If the card already owns the resources, the function
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succeeds. vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested
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calls are supported (a per-resource counter is maintained).
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Set the target device of this client.
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int pci_device_vgaarb_set_target (struct pci_device *dev);
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For instance, in x86 if two devices on the same bus want to lock different
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resources, both will succeed (lock). If devices are in different buses and
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trying to lock different resources, only the first who tried succeeds.
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int pci_device_vgaarb_lock (void);
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int pci_device_vgaarb_trylock (void);
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Unlock resources of device.
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int pci_device_vgaarb_unlock (void);
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Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA
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Memory, both, or none. All cards default to both, the card driver (fbdev for
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example) should tell the arbiter if it has disabled legacy decoding, so the
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card can be left out of the arbitration process (and can be safe to take
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interrupts at any time.
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int pci_device_vgaarb_decodes (int new_vgaarb_rsrc);
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Connects to the arbiter device, allocates the struct
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int pci_device_vgaarb_init (void);
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Close the connection
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void pci_device_vgaarb_fini (void);
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I.3 xf86VGAArbiter (X server implementation)
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--------------------------------------------
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(TODO)
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X server basically wraps all the functions that touch VGA registers somehow.
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II. Credits
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===========
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Benjamin Herrenschmidt (IBM?) started this work when he discussed such design
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with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and
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Tiago Vignatti (both of C3SL/Federal University of Paran<EFBFBD>) proceeded his work
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enhancing the kernel code to adapt as a kernel module and also did the
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implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave
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Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree.
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III. References
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==============
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[0] http://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347
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[1] http://lists.freedesktop.org/archives/xorg/2005-March/006663.html
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[2] http://lists.freedesktop.org/archives/xorg/2005-March/006745.html
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[3] http://lists.freedesktop.org/archives/xorg/2007-October/029507.html
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