Based on kernel version 4.8. Page generated on 2016-10-06 23:19 EST.
1 2 VGA Arbiter 3 =========== 4 5 Graphic devices are accessed through ranges in I/O or memory space. While most 6 modern devices allow relocation of such ranges, some "Legacy" VGA devices 7 implemented on PCI will typically have the same "hard-decoded" addresses as 8 they did on ISA. For more details see "PCI Bus Binding to IEEE Std 1275-1994 9 Standard for Boot (Initialization Configuration) Firmware Revision 2.1" 10 Section 7, Legacy Devices. 11 12 The Resource Access Control (RAC) module inside the X server [0] existed for 13 the legacy VGA arbitration task (besides other bus management tasks) when more 14 than one legacy device co-exists on the same machine. But the problem happens 15 when these devices are trying to be accessed by different userspace clients 16 (e.g. two server in parallel). Their address assignments conflict. Moreover, 17 ideally, being a userspace application, it is not the role of the X server to 18 control bus resources. Therefore an arbitration scheme outside of the X server 19 is needed to control the sharing of these resources. This document introduces 20 the operation of the VGA arbiter implemented for the Linux kernel. 21 22 ---------------------------------------------------------------------------- 23 24 I. Details and Theory of Operation 25 I.1 vgaarb 26 I.2 libpciaccess 27 I.3 xf86VGAArbiter (X server implementation) 28 II. Credits 29 III.References 30 31 32 I. Details and Theory of Operation 33 ================================== 34 35 I.1 vgaarb 36 ---------- 37 38 The vgaarb is a module of the Linux Kernel. When it is initially loaded, it 39 scans all PCI devices and adds the VGA ones inside the arbitration. The 40 arbiter then enables/disables the decoding on different devices of the VGA 41 legacy instructions. Devices which do not want/need to use the arbiter may 42 explicitly tell it by calling vga_set_legacy_decoding(). 43 44 The kernel exports a char device interface (/dev/vga_arbiter) to the clients, 45 which 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 84 Note about locks: 85 86 The driver keeps track of which user has which locks on which card. It 87 supports stacking, like the kernel one. This complexifies the implementation 88 a bit, but makes the arbiter more tolerant to user space problems and able 89 to properly cleanup in all cases when a process dies. 90 Currently, a max of 16 cards can have locks simultaneously issued from 91 user space for a given user (file descriptor instance) of the arbiter. 92 93 In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to 94 notify them being added/removed in the system and automatically added/removed 95 in the arbiter. 96 97 There is also an in-kernel API of the arbiter in case DRM, vgacon, or other 98 drivers want to use it. 99 100 101 I.2 libpciaccess 102 ---------------- 103 104 To use the vga arbiter char device it was implemented an API inside the 105 libpciaccess library. One field was added to struct pci_device (each device 106 on the system): 107 108 /* the type of resource decoded by the device */ 109 int vgaarb_rsrc; 110 111 Besides 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; 117 118 119 The vga_count is used to track how many cards are being arbitrated, so for 120 instance, if there is only one card, then it can completely escape arbitration. 121 122 123 These functions below acquire VGA resources for the given card and mark those 124 resources as locked. If the resources requested are "normal" (and not legacy) 125 resources, the arbiter will first check whether the card is doing legacy 126 decoding for that type of resource. If yes, the lock is "converted" into a 127 legacy resource lock. The arbiter will first look for all VGA cards that 128 might conflict and disable their IOs and/or Memory access, including VGA 129 forwarding on P2P bridges if necessary, so that the requested resources can 130 be used. Then, the card is marked as locking these resources and the IO and/or 131 Memory access is enabled on the card (including VGA forwarding on parent 132 P2P bridges if any). In the case of vga_arb_lock(), the function will block 133 if some conflicting card is already locking one of the required resources (or 134 any resource on a different bus segment, since P2P bridges don't differentiate 135 VGA memory and IO afaik). If the card already owns the resources, the function 136 succeeds. vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested 137 calls are supported (a per-resource counter is maintained). 138 139 140 Set the target device of this client. 141 int pci_device_vgaarb_set_target (struct pci_device *dev); 142 143 144 For instance, in x86 if two devices on the same bus want to lock different 145 resources, both will succeed (lock). If devices are in different buses and 146 trying to lock different resources, only the first who tried succeeds. 147 int pci_device_vgaarb_lock (void); 148 int pci_device_vgaarb_trylock (void); 149 150 Unlock resources of device. 151 int pci_device_vgaarb_unlock (void); 152 153 Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA 154 Memory, both, or none. All cards default to both, the card driver (fbdev for 155 example) should tell the arbiter if it has disabled legacy decoding, so the 156 card can be left out of the arbitration process (and can be safe to take 157 interrupts at any time. 158 int pci_device_vgaarb_decodes (int new_vgaarb_rsrc); 159 160 Connects to the arbiter device, allocates the struct 161 int pci_device_vgaarb_init (void); 162 163 Close the connection 164 void pci_device_vgaarb_fini (void); 165 166 167 I.3 xf86VGAArbiter (X server implementation) 168 -------------------------------------------- 169 170 (TODO) 171 172 X server basically wraps all the functions that touch VGA registers somehow. 173 174 175 II. Credits 176 =========== 177 178 Benjamin Herrenschmidt (IBM?) started this work when he discussed such design 179 with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and 180 Tiago Vignatti (both of C3SL/Federal University of Paraná) proceeded his work 181 enhancing the kernel code to adapt as a kernel module and also did the 182 implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave 183 Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree. 184 185 186 III. 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
