linux/Documentation/vgaarbiter.txt
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   2VGA Arbiter
   3===========
   4
   5Graphic devices are accessed through ranges in I/O or memory space. While most
   6modern devices allow relocation of such ranges, some "Legacy" VGA devices
   7implemented on PCI will typically have the same "hard-decoded" addresses as
   8they did on ISA. For more details see "PCI Bus Binding to IEEE Std 1275-1994
   9Standard for Boot (Initialization Configuration) Firmware Revision 2.1"
  10Section 7, Legacy Devices.
  11
  12The Resource Access Control (RAC) module inside the X server [0] existed for
  13the legacy VGA arbitration task (besides other bus management tasks) when more
  14than one legacy device co-exists on the same machine. But the problem happens
  15when these devices are trying to be accessed by different userspace clients
  16(e.g. two server in parallel). Their address assignments conflict. Moreover,
  17ideally, being a userspace application, it is not the role of the X server to
  18control bus resources. Therefore an arbitration scheme outside of the X server
  19is needed to control the sharing of these resources. This document introduces
  20the operation of the VGA arbiter implemented for the Linux kernel.
  21
  22----------------------------------------------------------------------------
  23
  24I.  Details and Theory of Operation
  25        I.1 vgaarb
  26        I.2 libpciaccess
  27        I.3 xf86VGAArbiter (X server implementation)
  28II. Credits
  29III.References
  30
  31
  32I. Details and Theory of Operation
  33==================================
  34
  35I.1 vgaarb
  36----------
  37
  38The vgaarb is a module of the Linux Kernel. When it is initially loaded, it
  39scans all PCI devices and adds the VGA ones inside the arbitration. The
  40arbiter then enables/disables the decoding on different devices of the VGA
  41legacy instructions. Devices which do not want/need to use the arbiter may
  42explicitly tell it by calling vga_set_legacy_decoding().
  43
  44The kernel exports a char device interface (/dev/vga_arbiter) to the clients,
  45which has the following semantics:
  46
  47 open       : open user instance of the arbiter. By default, it's attached to
  48              the default VGA device of the system.
  49
  50 close      : close user instance. Release locks made by the user
  51
  52 read       : return a string indicating the status of the target like:
  53
  54              "<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)"
  55
  56              An IO state string is of the form {io,mem,io+mem,none}, mc and
  57              ic are respectively mem and io lock counts (for debugging/
  58              diagnostic only). "decodes" indicate what the card currently
  59              decodes, "owns" indicates what is currently enabled on it, and
  60              "locks" indicates what is locked by this card. If the card is
  61              unplugged, we get "invalid" then for card_ID and an -ENODEV
  62              error is returned for any command until a new card is targeted.
  63
  64
  65 write       : write a command to the arbiter. List of commands:
  66
  67  target <card_ID>   : switch target to card <card_ID> (see below)
  68  lock <io_state>    : acquires locks on target ("none" is an invalid io_state)
  69  trylock <io_state> : non-blocking acquire locks on target (returns EBUSY if
  70                       unsuccessful)
  71  unlock <io_state>  : release locks on target
  72  unlock all         : release all locks on target held by this user (not
  73                       implemented yet)
  74  decodes <io_state> : set the legacy decoding attributes for the card
  75
  76  poll               : event if something changes on any card (not just the
  77                       target)
  78
  79  card_ID is of the form "PCI:domain:bus:dev.fn". It can be set to "default"
  80  to go back to the system default card (TODO: not implemented yet). Currently,
  81  only PCI is supported as a prefix, but the userland API may support other bus
  82  types in the future, even if the current kernel implementation doesn't.
  83
  84Note about locks:
  85
  86The driver keeps track of which user has which locks on which card. It
  87supports stacking, like the kernel one. This complexifies the implementation
  88a bit, but makes the arbiter more tolerant to user space problems and able
  89to properly cleanup in all cases when a process dies.
  90Currently, a max of 16 cards can have locks simultaneously issued from
  91user space for a given user (file descriptor instance) of the arbiter.
  92
  93In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to
  94notify them being added/removed in the system and automatically added/removed
  95in the arbiter.
  96
  97There is also an in-kernel API of the arbiter in case DRM, vgacon, or other
  98drivers want to use it.
  99
 100
 101I.2 libpciaccess
 102----------------
 103
 104To use the vga arbiter char device it was implemented an API inside the
 105libpciaccess library. One field was added to struct pci_device (each device
 106on the system):
 107
 108    /* the type of resource decoded by the device */
 109    int vgaarb_rsrc;
 110
 111Besides it, in pci_system were added:
 112
 113    int vgaarb_fd;
 114    int vga_count;
 115    struct pci_device *vga_target;
 116    struct pci_device *vga_default_dev;
 117
 118
 119The vga_count is used to track how many cards are being arbitrated, so for
 120instance, if there is only one card, then it can completely escape arbitration.
 121
 122
 123These functions below acquire VGA resources for the given card and mark those
 124resources as locked. If the resources requested are "normal" (and not legacy)
 125resources, the arbiter will first check whether the card is doing legacy
 126decoding for that type of resource. If yes, the lock is "converted" into a
 127legacy resource lock. The arbiter will first look for all VGA cards that
 128might conflict and disable their IOs and/or Memory access, including VGA
 129forwarding on P2P bridges if necessary, so that the requested resources can
 130be used. Then, the card is marked as locking these resources and the IO and/or
 131Memory access is enabled on the card (including VGA forwarding on parent
 132P2P bridges if any). In the case of vga_arb_lock(), the function will block
 133if some conflicting card is already locking one of the required resources (or
 134any resource on a different bus segment, since P2P bridges don't differentiate
 135VGA memory and IO afaik). If the card already owns the resources, the function
 136succeeds.  vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested
 137calls are supported (a per-resource counter is maintained).
 138
 139
 140Set the target device of this client.
 141    int  pci_device_vgaarb_set_target   (struct pci_device *dev);
 142
 143
 144For instance, in x86 if two devices on the same bus want to lock different
 145resources, both will succeed (lock). If devices are in different buses and
 146trying to lock different resources, only the first who tried succeeds.
 147    int  pci_device_vgaarb_lock         (void);
 148    int  pci_device_vgaarb_trylock      (void);
 149
 150Unlock resources of device.
 151    int  pci_device_vgaarb_unlock       (void);
 152
 153Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA
 154Memory, both, or none. All cards default to both, the card driver (fbdev for
 155example) should tell the arbiter if it has disabled legacy decoding, so the
 156card can be left out of the arbitration process (and can be safe to take
 157interrupts at any time.
 158    int  pci_device_vgaarb_decodes      (int new_vgaarb_rsrc);
 159
 160Connects to the arbiter device, allocates the struct
 161    int  pci_device_vgaarb_init         (void);
 162
 163Close the connection
 164    void pci_device_vgaarb_fini         (void);
 165
 166
 167I.3 xf86VGAArbiter (X server implementation)
 168--------------------------------------------
 169
 170(TODO)
 171
 172X server basically wraps all the functions that touch VGA registers somehow.
 173
 174
 175II. Credits
 176===========
 177
 178Benjamin Herrenschmidt (IBM?) started this work when he discussed such design
 179with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and
 180Tiago Vignatti (both of C3SL/Federal University of ParanĂ¡) proceeded his work
 181enhancing the kernel code to adapt as a kernel module and also did the
 182implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave
 183Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree.
 184
 185
 186III. References
 187==============
 188
 189[0] http://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347
 190[1] http://lists.freedesktop.org/archives/xorg/2005-March/006663.html
 191[2] http://lists.freedesktop.org/archives/xorg/2005-March/006745.html
 192[3] http://lists.freedesktop.org/archives/xorg/2007-October/029507.html
 193
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