[lkml]   [2009]   [Jul]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    Patch in this message
    Subject[PATCH 2/3] vga: drops a documentation regarding the VGA arbiter
    Signed-off-by: Tiago Vignatti <>
    Documentation/vgaarbiter.txt | 197 ++++++++++++++++++++++++++++++++++++++++++
    1 files changed, 197 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/vgaarbiter.txt

    diff --git a/Documentation/vgaarbiter.txt b/Documentation/vgaarbiter.txt
    new file mode 100644
    index 0000000..37d3126
    --- /dev/null
    +++ b/Documentation/vgaarbiter.txt
    @@ -0,0 +1,197 @@
    +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 currently does
    +the task of arbitration 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. 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. Open Issues / Bugs
    +VI. 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 add the VGA ones inside the arbitration. The arbiter
    +then enables/disables the decoding of VGA legacy instructions by different
    +devices which are trying to use them on the same machine. The device which
    +does not want/need to use the arbiter may explicitly tell it by calling
    +vga_arb_decodes() from the VGA library function. Driver must take a special
    +care here.
    +Basically the kernel exports a char device interface (/dev/vga_arbiter) to the
    +clients. It has the usual 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.
    +If the device is hot-{un,}plugged, there is a hook inside the module 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 a serie of functions
    +inside the pciaccess library. Two fields were added to struct pci_device for
    +this be possible:
    + /* the type of resource decoded by the device */
    + int vgaarb_rsrc;
    + /* the file descriptor (/dev/vga_arbiter) */
    + int vgaarb_fd;
    +and the functions:
    +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 (struct pci_device *dev);
    + int pci_device_vgaarb_trylock (struct pci_device *dev);
    +Unlock resources of device.
    + int pci_device_vgaarb_unlock (struct pci_device *dev);
    +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 (struct pci_device *dev);
    +Connects to the arbiter device, allocates the struct
    + int pci_device_vgaarb_init (struct pci_device *dev);
    +Close the connection
    + void pci_device_vgaarb_fini (struct pci_device *dev);
    +I.3 xf86VGAArbiter (X server implementation)
    +This work will probably remove the RAC (Resource Access Control) code inside
    +Xorg DDX. Complain: RAC is non-OS dependent. VGA Arbiter is.
    +II. Open Issues / Bugs
    +Current status:
    + - two X servers is parallel works (multiseat style)
    + - non-bottable card works
    + - secondary card works
    + - xinemara-like _doesn't_ works (need to trace more paths paths that touch
    + the registers and bracket with lock/unlock)
    +III. 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 is
    +finally trying to push such work upstream.
    +VI. References
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to
    More majordomo info at
    Please read the FAQ at

     \ /
      Last update: 2009-07-16 17:55    [W:0.034 / U:102.432 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site