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Source at commit b386be689295730688885552666ea40b2e639b14 created 11 years 11 months ago. By Maarten ter Huurne, Revert "MIPS: JZ4740: reset: Initialize hibernate wakeup counters." | |
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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 ParanĂ¡) 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 |
193 |
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