vgaarbiter.rst (8177B)
1=========== 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 22vgaarb kernel/userspace ABI 23--------------------------- 24 25The vgaarb is a module of the Linux Kernel. When it is initially loaded, it 26scans all PCI devices and adds the VGA ones inside the arbitration. The 27arbiter then enables/disables the decoding on different devices of the VGA 28legacy instructions. Devices which do not want/need to use the arbiter may 29explicitly tell it by calling vga_set_legacy_decoding(). 30 31The kernel exports a char device interface (/dev/vga_arbiter) to the clients, 32which has the following semantics: 33 34open 35 Opens a user instance of the arbiter. By default, it's attached to the 36 default VGA device of the system. 37 38close 39 Close a user instance. Release locks made by the user 40 41read 42 Return a string indicating the status of the target like: 43 44 "<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)" 45 46 An IO state string is of the form {io,mem,io+mem,none}, mc and 47 ic are respectively mem and io lock counts (for debugging/ 48 diagnostic only). "decodes" indicate what the card currently 49 decodes, "owns" indicates what is currently enabled on it, and 50 "locks" indicates what is locked by this card. If the card is 51 unplugged, we get "invalid" then for card_ID and an -ENODEV 52 error is returned for any command until a new card is targeted. 53 54 55write 56 Write a command to the arbiter. List of commands: 57 58 target <card_ID> 59 switch target to card <card_ID> (see below) 60 lock <io_state> 61 acquires locks on target ("none" is an invalid io_state) 62 trylock <io_state> 63 non-blocking acquire locks on target (returns EBUSY if 64 unsuccessful) 65 unlock <io_state> 66 release locks on target 67 unlock all 68 release all locks on target held by this user (not implemented 69 yet) 70 decodes <io_state> 71 set the legacy decoding attributes for the card 72 73 poll 74 event if something changes on any card (not just the target) 75 76 card_ID is of the form "PCI:domain:bus:dev.fn". It can be set to "default" 77 to go back to the system default card (TODO: not implemented yet). Currently, 78 only PCI is supported as a prefix, but the userland API may support other bus 79 types in the future, even if the current kernel implementation doesn't. 80 81Note about locks: 82 83The driver keeps track of which user has which locks on which card. It 84supports stacking, like the kernel one. This complexifies the implementation 85a bit, but makes the arbiter more tolerant to user space problems and able 86to properly cleanup in all cases when a process dies. 87Currently, a max of 16 cards can have locks simultaneously issued from 88user space for a given user (file descriptor instance) of the arbiter. 89 90In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to 91notify them being added/removed in the system and automatically added/removed 92in the arbiter. 93 94There is also an in-kernel API of the arbiter in case DRM, vgacon, or other 95drivers want to use it. 96 97In-kernel interface 98------------------- 99 100.. kernel-doc:: include/linux/vgaarb.h 101 :internal: 102 103.. kernel-doc:: drivers/pci/vgaarb.c 104 :export: 105 106libpciaccess 107------------ 108 109To use the vga arbiter char device it was implemented an API inside the 110libpciaccess library. One field was added to struct pci_device (each device 111on the system):: 112 113 /* the type of resource decoded by the device */ 114 int vgaarb_rsrc; 115 116Besides it, in pci_system were added:: 117 118 int vgaarb_fd; 119 int vga_count; 120 struct pci_device *vga_target; 121 struct pci_device *vga_default_dev; 122 123The vga_count is used to track how many cards are being arbitrated, so for 124instance, if there is only one card, then it can completely escape arbitration. 125 126These functions below acquire VGA resources for the given card and mark those 127resources as locked. If the resources requested are "normal" (and not legacy) 128resources, the arbiter will first check whether the card is doing legacy 129decoding for that type of resource. If yes, the lock is "converted" into a 130legacy resource lock. The arbiter will first look for all VGA cards that 131might conflict and disable their IOs and/or Memory access, including VGA 132forwarding on P2P bridges if necessary, so that the requested resources can 133be used. Then, the card is marked as locking these resources and the IO and/or 134Memory access is enabled on the card (including VGA forwarding on parent 135P2P bridges if any). In the case of vga_arb_lock(), the function will block 136if some conflicting card is already locking one of the required resources (or 137any resource on a different bus segment, since P2P bridges don't differentiate 138VGA memory and IO afaik). If the card already owns the resources, the function 139succeeds. vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested 140calls are supported (a per-resource counter is maintained). 141 142Set the target device of this client. :: 143 144 int pci_device_vgaarb_set_target (struct pci_device *dev); 145 146For instance, in x86 if two devices on the same bus want to lock different 147resources, both will succeed (lock). If devices are in different buses and 148trying to lock different resources, only the first who tried succeeds. :: 149 150 int pci_device_vgaarb_lock (void); 151 int pci_device_vgaarb_trylock (void); 152 153Unlock resources of device. :: 154 155 int pci_device_vgaarb_unlock (void); 156 157Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA 158Memory, both, or none. All cards default to both, the card driver (fbdev for 159example) should tell the arbiter if it has disabled legacy decoding, so the 160card can be left out of the arbitration process (and can be safe to take 161interrupts at any time. :: 162 163 int pci_device_vgaarb_decodes (int new_vgaarb_rsrc); 164 165Connects to the arbiter device, allocates the struct :: 166 167 int pci_device_vgaarb_init (void); 168 169Close the connection :: 170 171 void pci_device_vgaarb_fini (void); 172 173xf86VGAArbiter (X server implementation) 174---------------------------------------- 175 176X server basically wraps all the functions that touch VGA registers somehow. 177 178References 179---------- 180 181Benjamin Herrenschmidt (IBM?) started this work when he discussed such design 182with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and 183Tiago Vignatti (both of C3SL/Federal University of Paraná) proceeded his work 184enhancing the kernel code to adapt as a kernel module and also did the 185implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave 186Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree. 187 1880) https://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347 1891) https://lists.freedesktop.org/archives/xorg/2005-March/006663.html 1902) https://lists.freedesktop.org/archives/xorg/2005-March/006745.html 1913) https://lists.freedesktop.org/archives/xorg/2007-October/029507.html