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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 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
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