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 vgaarb kernel/userspace ABI
23 ---------------------------
24
25 The vgaarb is a module of the Linux Kernel. When it is initially loaded, it
26 scans all PCI devices and adds the VGA ones inside the arbitration. The
27 arbiter then enables/disables the decoding on different devices of the VGA
28 legacy instructions. Devices which do not want/need to use the arbiter may
29 explicitly tell it by calling vga_set_legacy_decoding().
30
31 The kernel exports a char device interface (/dev/vga_arbiter) to the clients,
32 which has the following semantics:
33
34 open
35 Opens a user instance of the arbiter. By default, it's attached to the
36 default VGA device of the system.
37
38 close
39 Close a user instance. Release locks made by the user
40
41 read
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
55 write
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
81 Note about locks:
82
83 The driver keeps track of which user has which locks on which card. It
84 supports stacking, like the kernel one. This complexifies the implementation
85 a bit, but makes the arbiter more tolerant to user space problems and able
86 to properly cleanup in all cases when a process dies.
87 Currently, a max of 16 cards can have locks simultaneously issued from
88 user space for a given user (file descriptor instance) of the arbiter.
89
90 In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to
91 notify them being added/removed in the system and automatically added/removed
92 in the arbiter.
93
94 There is also an in-kernel API of the arbiter in case DRM, vgacon, or other
95 drivers want to use it.
96
97 In-kernel interface
98 -------------------
99
100 .. kernel-doc:: include/linux/vgaarb.h
101 :internal:
102
103 .. kernel-doc:: drivers/pci/vgaarb.c
104 :export:
105
106 libpciaccess
107 ------------
108
109 To use the vga arbiter char device it was implemented an API inside the
110 libpciaccess library. One field was added to struct pci_device (each device
111 on the system)::
112
113 /* the type of resource decoded by the device */
114 int vgaarb_rsrc;
115
116 Besides 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
123 The vga_count is used to track how many cards are being arbitrated, so for
124 instance, if there is only one card, then it can completely escape arbitration.
125
126 These functions below acquire VGA resources for the given card and mark those
127 resources as locked. If the resources requested are "normal" (and not legacy)
128 resources, the arbiter will first check whether the card is doing legacy
129 decoding for that type of resource. If yes, the lock is "converted" into a
130 legacy resource lock. The arbiter will first look for all VGA cards that
131 might conflict and disable their IOs and/or Memory access, including VGA
132 forwarding on P2P bridges if necessary, so that the requested resources can
133 be used. Then, the card is marked as locking these resources and the IO and/or
134 Memory access is enabled on the card (including VGA forwarding on parent
135 P2P bridges if any). In the case of vga_arb_lock(), the function will block
136 if some conflicting card is already locking one of the required resources (or
137 any resource on a different bus segment, since P2P bridges don't differentiate
138 VGA memory and IO afaik). If the card already owns the resources, the function
139 succeeds. vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested
140 calls are supported (a per-resource counter is maintained).
141
142 Set the target device of this client. ::
143
144 int pci_device_vgaarb_set_target (struct pci_device *dev);
145
146 For instance, in x86 if two devices on the same bus want to lock different
147 resources, both will succeed (lock). If devices are in different buses and
148 trying 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
153 Unlock resources of device. ::
154
155 int pci_device_vgaarb_unlock (void);
156
157 Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA
158 Memory, both, or none. All cards default to both, the card driver (fbdev for
159 example) should tell the arbiter if it has disabled legacy decoding, so the
160 card can be left out of the arbitration process (and can be safe to take
161 interrupts at any time. ::
162
163 int pci_device_vgaarb_decodes (int new_vgaarb_rsrc);
164
165 Connects to the arbiter device, allocates the struct ::
166
167 int pci_device_vgaarb_init (void);
168
169 Close the connection ::
170
171 void pci_device_vgaarb_fini (void);
172
173 xf86VGAArbiter (X server implementation)
174 ----------------------------------------
175
176 X server basically wraps all the functions that touch VGA registers somehow.
177
178 References
179 ----------
180
181 Benjamin Herrenschmidt (IBM?) started this work when he discussed such design
182 with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and
183 Tiago Vignatti (both of C3SL/Federal University of Paraná) proceeded his work
184 enhancing the kernel code to adapt as a kernel module and also did the
185 implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave
186 Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree.
187
188 0) https://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347
189 1) https://lists.freedesktop.org/archives/xorg/2005-March/006663.html
190 2) https://lists.freedesktop.org/archives/xorg/2005-March/006745.html
191 3) https://lists.freedesktop.org/archives/xorg/2007-October/029507.html
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