Root/
Source at commit be977234bfb4a6dca8a39e7c52165e4cd536ad71 created 12 years 9 months ago. By Lars-Peter Clausen, jz4740: Fix compile error | |
---|---|
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 an userspace application, it is not the role of the the X |
18 | server to control bus resources. Therefore an arbitration scheme outside of |
19 | the X server is needed to control the sharing of these resources. This |
20 | document introduces the operation of the VGA arbiter implemented for Linux |
21 | kernel. |
22 | |
23 | ---------------------------------------------------------------------------- |
24 | |
25 | I. Details and Theory of Operation |
26 | I.1 vgaarb |
27 | I.2 libpciaccess |
28 | I.3 xf86VGAArbiter (X server implementation) |
29 | II. Credits |
30 | III.References |
31 | |
32 | |
33 | I. Details and Theory of Operation |
34 | ================================== |
35 | |
36 | I.1 vgaarb |
37 | ---------- |
38 | |
39 | The vgaarb is a module of the Linux Kernel. When it is initially loaded, it |
40 | scans all PCI devices and adds the VGA ones inside the arbitration. The |
41 | arbiter then enables/disables the decoding on different devices of the VGA |
42 | legacy instructions. Device which do not want/need to use the arbiter may |
43 | explicitly tell it by calling vga_set_legacy_decoding(). |
44 | |
45 | The kernel exports a char device interface (/dev/vga_arbiter) to the clients, |
46 | which has the following semantics: |
47 | |
48 | open : open user instance of the arbiter. By default, it's attached to |
49 | the default VGA device of the system. |
50 | |
51 | close : close user instance. Release locks made by the user |
52 | |
53 | read : return a string indicating the status of the target like: |
54 | |
55 | "<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)" |
56 | |
57 | An IO state string is of the form {io,mem,io+mem,none}, mc and |
58 | ic are respectively mem and io lock counts (for debugging/ |
59 | diagnostic only). "decodes" indicate what the card currently |
60 | decodes, "owns" indicates what is currently enabled on it, and |
61 | "locks" indicates what is locked by this card. If the card is |
62 | unplugged, we get "invalid" then for card_ID and an -ENODEV |
63 | error is returned for any command until a new card is targeted. |
64 | |
65 | |
66 | write : write a command to the arbiter. List of commands: |
67 | |
68 | target <card_ID> : switch target to card <card_ID> (see below) |
69 | lock <io_state> : acquires locks on target ("none" is an invalid io_state) |
70 | trylock <io_state> : non-blocking acquire locks on target (returns EBUSY if |
71 | unsuccessful) |
72 | unlock <io_state> : release locks on target |
73 | unlock all : release all locks on target held by this user (not |
74 | implemented yet) |
75 | decodes <io_state> : set the legacy decoding attributes for the card |
76 | |
77 | poll : event if something changes on any card (not just the |
78 | target) |
79 | |
80 | card_ID is of the form "PCI:domain:bus:dev.fn". It can be set to "default" |
81 | to go back to the system default card (TODO: not implemented yet). Currently, |
82 | only PCI is supported as a prefix, but the userland API may support other bus |
83 | types in the future, even if the current kernel implementation doesn't. |
84 | |
85 | Note about locks: |
86 | |
87 | The driver keeps track of which user has which locks on which card. It |
88 | supports stacking, like the kernel one. This complexifies the implementation |
89 | a bit, but makes the arbiter more tolerant to user space problems and able |
90 | to properly cleanup in all cases when a process dies. |
91 | Currently, a max of 16 cards can have locks simultaneously issued from |
92 | user space for a given user (file descriptor instance) of the arbiter. |
93 | |
94 | In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to |
95 | notify them being added/removed in the system and automatically added/removed |
96 | in the arbiter. |
97 | |
98 | There's also a in-kernel API of the arbiter in the case of DRM, vgacon and |
99 | others which may use the arbiter. |
100 | |
101 | |
102 | I.2 libpciaccess |
103 | ---------------- |
104 | |
105 | To use the vga arbiter char device it was implemented an API inside the |
106 | libpciaccess library. One field was added to struct pci_device (each device |
107 | on the system): |
108 | |
109 | /* the type of resource decoded by the device */ |
110 | int vgaarb_rsrc; |
111 | |
112 | Besides it, in pci_system were added: |
113 | |
114 | int vgaarb_fd; |
115 | int vga_count; |
116 | struct pci_device *vga_target; |
117 | struct pci_device *vga_default_dev; |
118 | |
119 | |
120 | The vga_count is usually need to keep informed how many cards are being |
121 | arbitrated, so for instance if there's only one then it can totally escape the |
122 | scheme. |
123 | |
124 | |
125 | These functions below acquire VGA resources for the given card and mark those |
126 | resources as locked. If the resources requested are "normal" (and not legacy) |
127 | resources, the arbiter will first check whether the card is doing legacy |
128 | decoding for that type of resource. If yes, the lock is "converted" into a |
129 | legacy resource lock. The arbiter will first look for all VGA cards that |
130 | might conflict and disable their IOs and/or Memory access, including VGA |
131 | forwarding on P2P bridges if necessary, so that the requested resources can |
132 | be used. Then, the card is marked as locking these resources and the IO and/or |
133 | Memory access is enabled on the card (including VGA forwarding on parent |
134 | P2P bridges if any). In the case of vga_arb_lock(), the function will block |
135 | if some conflicting card is already locking one of the required resources (or |
136 | any resource on a different bus segment, since P2P bridges don't differentiate |
137 | VGA memory and IO afaik). If the card already owns the resources, the function |
138 | succeeds. vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested |
139 | calls are supported (a per-resource counter is maintained). |
140 | |
141 | |
142 | Set the target device of this client. |
143 | int pci_device_vgaarb_set_target (struct pci_device *dev); |
144 | |
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 | int pci_device_vgaarb_lock (void); |
150 | int pci_device_vgaarb_trylock (void); |
151 | |
152 | Unlock resources of device. |
153 | int pci_device_vgaarb_unlock (void); |
154 | |
155 | Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA |
156 | Memory, both, or none. All cards default to both, the card driver (fbdev for |
157 | example) should tell the arbiter if it has disabled legacy decoding, so the |
158 | card can be left out of the arbitration process (and can be safe to take |
159 | interrupts at any time. |
160 | int pci_device_vgaarb_decodes (int new_vgaarb_rsrc); |
161 | |
162 | Connects to the arbiter device, allocates the struct |
163 | int pci_device_vgaarb_init (void); |
164 | |
165 | Close the connection |
166 | void pci_device_vgaarb_fini (void); |
167 | |
168 | |
169 | I.3 xf86VGAArbiter (X server implementation) |
170 | -------------------------------------------- |
171 | |
172 | (TODO) |
173 | |
174 | X server basically wraps all the functions that touch VGA registers somehow. |
175 | |
176 | |
177 | II. Credits |
178 | =========== |
179 | |
180 | Benjamin Herrenschmidt (IBM?) started this work when he discussed such design |
181 | with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and |
182 | Tiago Vignatti (both of C3SL/Federal University of Paraná) proceeded his work |
183 | enhancing the kernel code to adapt as a kernel module and also did the |
184 | implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave |
185 | Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree. |
186 | |
187 | |
188 | III. References |
189 | ============== |
190 | |
191 | [0] http://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347 |
192 | [1] http://lists.freedesktop.org/archives/xorg/2005-March/006663.html |
193 | [2] http://lists.freedesktop.org/archives/xorg/2005-March/006745.html |
194 | [3] http://lists.freedesktop.org/archives/xorg/2007-October/029507.html |
195 |
Branches:
ben-wpan
ben-wpan-stefan
javiroman/ks7010
jz-2.6.34
jz-2.6.34-rc5
jz-2.6.34-rc6
jz-2.6.34-rc7
jz-2.6.35
jz-2.6.36
jz-2.6.37
jz-2.6.38
jz-2.6.39
jz-3.0
jz-3.1
jz-3.11
jz-3.12
jz-3.13
jz-3.15
jz-3.16
jz-3.18-dt
jz-3.2
jz-3.3
jz-3.4
jz-3.5
jz-3.6
jz-3.6-rc2-pwm
jz-3.9
jz-3.9-clk
jz-3.9-rc8
jz47xx
jz47xx-2.6.38
master
Tags:
od-2011-09-04
od-2011-09-18
v2.6.34-rc5
v2.6.34-rc6
v2.6.34-rc7
v3.9