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0737c4e PCI/VGA: add VGA arbitration documentation
Tiago Vignatti authored
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,
f14d8d3 @nikai3d Documentation: fix vgaarbiter.txt typos etc.
nikai3d authored
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.
0737c4e PCI/VGA: add VGA arbitration documentation
Tiago Vignatti authored
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
f14d8d3 @nikai3d Documentation: fix vgaarbiter.txt typos etc.
nikai3d authored
41 legacy instructions. Devices which do not want/need to use the arbiter may
0737c4e PCI/VGA: add VGA arbitration documentation
Tiago Vignatti authored
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
f14d8d3 @nikai3d Documentation: fix vgaarbiter.txt typos etc.
nikai3d authored
97 There is also an in-kernel API of the arbiter in case DRM, vgacon, or other
98 drivers want to use it.
0737c4e PCI/VGA: add VGA arbitration documentation
Tiago Vignatti authored
99
100
101 I.2 libpciaccess
102 ----------------
103
104 To use the vga arbiter char device it was implemented an API inside the
15293df @winspool vgaarbiter: fix a typo in the vgaarbiter Documentation
winspool authored
105 libpciaccess library. One field was added to struct pci_device (each device
0737c4e PCI/VGA: add VGA arbitration documentation
Tiago Vignatti authored
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
f14d8d3 @nikai3d Documentation: fix vgaarbiter.txt typos etc.
nikai3d authored
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.
0737c4e PCI/VGA: add VGA arbitration documentation
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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
d36b691 misc latin1 to utf8 conversions
Al Viro authored
180 Tiago Vignatti (both of C3SL/Federal University of Paraná) proceeded his work
0737c4e PCI/VGA: add VGA arbitration documentation
Tiago Vignatti authored
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
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