192 lines
8.0 KiB
ReStructuredText
192 lines
8.0 KiB
ReStructuredText
|
===========
|
||
|
|
VGA Arbiter
|
||
|
|
===========
|
||
|
|
|
||
|
|
Graphic devices are accessed through ranges in I/O or memory space. While most
|
||
|
|
modern devices allow relocation of such ranges, some "Legacy" VGA devices
|
||
|
|
implemented on PCI will typically have the same "hard-decoded" addresses as
|
||
|
|
they did on ISA. For more details see "PCI Bus Binding to IEEE Std 1275-1994
|
||
|
|
Standard for Boot (Initialization Configuration) Firmware Revision 2.1"
|
||
|
|
Section 7, Legacy Devices.
|
||
|
|
|
||
|
|
The Resource Access Control (RAC) module inside the X server [0] existed for
|
||
|
|
the legacy VGA arbitration task (besides other bus management tasks) when more
|
||
|
|
than one legacy device co-exists on the same machine. But the problem happens
|
||
|
|
when these devices are trying to be accessed by different userspace clients
|
||
|
|
(e.g. two server in parallel). Their address assignments conflict. Moreover,
|
||
|
|
ideally, being a userspace application, it is not the role of the X server to
|
||
|
|
control bus resources. Therefore an arbitration scheme outside of the X server
|
||
|
|
is needed to control the sharing of these resources. This document introduces
|
||
|
|
the operation of the VGA arbiter implemented for the Linux kernel.
|
||
|
|
|
||
|
|
vgaarb kernel/userspace ABI
|
||
|
|
---------------------------
|
||
|
|
|
||
|
|
The vgaarb is a module of the Linux Kernel. When it is initially loaded, it
|
||
|
|
scans all PCI devices and adds the VGA ones inside the arbitration. The
|
||
|
|
arbiter then enables/disables the decoding on different devices of the VGA
|
||
|
|
legacy instructions. Devices which do not want/need to use the arbiter may
|
||
|
|
explicitly tell it by calling vga_set_legacy_decoding().
|
||
|
|
|
||
|
|
The kernel exports a char device interface (/dev/vga_arbiter) to the clients,
|
||
|
|
which has the following semantics:
|
||
|
|
|
||
|
|
open
|
||
|
|
Opens a user instance of the arbiter. By default, it's attached to the
|
||
|
|
default VGA device of the system.
|
||
|
|
|
||
|
|
close
|
||
|
|
Close a user instance. Release locks made by the user
|
||
|
|
|
||
|
|
read
|
||
|
|
Return a string indicating the status of the target like:
|
||
|
|
|
||
|
|
"<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)"
|
||
|
|
|
||
|
|
An IO state string is of the form {io,mem,io+mem,none}, mc and
|
||
|
|
ic are respectively mem and io lock counts (for debugging/
|
||
|
|
diagnostic only). "decodes" indicate what the card currently
|
||
|
|
decodes, "owns" indicates what is currently enabled on it, and
|
||
|
|
"locks" indicates what is locked by this card. If the card is
|
||
|
|
unplugged, we get "invalid" then for card_ID and an -ENODEV
|
||
|
|
error is returned for any command until a new card is targeted.
|
||
|
|
|
||
|
|
|
||
|
|
write
|
||
|
|
Write a command to the arbiter. List of commands:
|
||
|
|
|
||
|
|
target <card_ID>
|
||
|
|
switch target to card <card_ID> (see below)
|
||
|
|
lock <io_state>
|
||
|
|
acquires locks on target ("none" is an invalid io_state)
|
||
|
|
trylock <io_state>
|
||
|
|
non-blocking acquire locks on target (returns EBUSY if
|
||
|
|
unsuccessful)
|
||
|
|
unlock <io_state>
|
||
|
|
release locks on target
|
||
|
|
unlock all
|
||
|
|
release all locks on target held by this user (not implemented
|
||
|
|
yet)
|
||
|
|
decodes <io_state>
|
||
|
|
set the legacy decoding attributes for the card
|
||
|
|
|
||
|
|
poll
|
||
|
|
event if something changes on any card (not just the target)
|
||
|
|
|
||
|
|
card_ID is of the form "PCI:domain:bus:dev.fn". It can be set to "default"
|
||
|
|
to go back to the system default card (TODO: not implemented yet). Currently,
|
||
|
|
only PCI is supported as a prefix, but the userland API may support other bus
|
||
|
|
types in the future, even if the current kernel implementation doesn't.
|
||
|
|
|
||
|
|
Note about locks:
|
||
|
|
|
||
|
|
The driver keeps track of which user has which locks on which card. It
|
||
|
|
supports stacking, like the kernel one. This complexifies the implementation
|
||
|
|
a bit, but makes the arbiter more tolerant to user space problems and able
|
||
|
|
to properly cleanup in all cases when a process dies.
|
||
|
|
Currently, a max of 16 cards can have locks simultaneously issued from
|
||
|
|
user space for a given user (file descriptor instance) of the arbiter.
|
||
|
|
|
||
|
|
In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to
|
||
|
|
notify them being added/removed in the system and automatically added/removed
|
||
|
|
in the arbiter.
|
||
|
|
|
||
|
|
There is also an in-kernel API of the arbiter in case DRM, vgacon, or other
|
||
|
|
drivers want to use it.
|
||
|
|
|
||
|
|
In-kernel interface
|
||
|
|
-------------------
|
||
|
|
|
||
|
|
.. kernel-doc:: include/linux/vgaarb.h
|
||
|
|
:internal:
|
||
|
|
|
||
|
|
.. kernel-doc:: drivers/gpu/vga/vgaarb.c
|
||
|
|
:export:
|
||
|
|
|
||
|
|
libpciaccess
|
||
|
|
------------
|
||
|
|
|
||
|
|
To use the vga arbiter char device it was implemented an API inside the
|
||
|
|
libpciaccess library. One field was added to struct pci_device (each device
|
||
|
|
on the system)::
|
||
|
|
|
||
|
|
/* the type of resource decoded by the device */
|
||
|
|
int vgaarb_rsrc;
|
||
|
|
|
||
|
|
Besides it, in pci_system were added::
|
||
|
|
|
||
|
|
int vgaarb_fd;
|
||
|
|
int vga_count;
|
||
|
|
struct pci_device *vga_target;
|
||
|
|
struct pci_device *vga_default_dev;
|
||
|
|
|
||
|
|
The vga_count is used to track how many cards are being arbitrated, so for
|
||
|
|
instance, if there is only one card, then it can completely escape arbitration.
|
||
|
|
|
||
|
|
These functions below acquire VGA resources for the given card and mark those
|
||
|
|
resources as locked. If the resources requested are "normal" (and not legacy)
|
||
|
|
resources, the arbiter will first check whether the card is doing legacy
|
||
|
|
decoding for that type of resource. If yes, the lock is "converted" into a
|
||
|
|
legacy resource lock. The arbiter will first look for all VGA cards that
|
||
|
|
might conflict and disable their IOs and/or Memory access, including VGA
|
||
|
|
forwarding on P2P bridges if necessary, so that the requested resources can
|
||
|
|
be used. Then, the card is marked as locking these resources and the IO and/or
|
||
|
|
Memory access is enabled on the card (including VGA forwarding on parent
|
||
|
|
P2P bridges if any). In the case of vga_arb_lock(), the function will block
|
||
|
|
if some conflicting card is already locking one of the required resources (or
|
||
|
|
any resource on a different bus segment, since P2P bridges don't differentiate
|
||
|
|
VGA memory and IO afaik). If the card already owns the resources, the function
|
||
|
|
succeeds. vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested
|
||
|
|
calls are supported (a per-resource counter is maintained).
|
||
|
|
|
||
|
|
Set the target device of this client. ::
|
||
|
|
|
||
|
|
int pci_device_vgaarb_set_target (struct pci_device *dev);
|
||
|
|
|
||
|
|
For instance, in x86 if two devices on the same bus want to lock different
|
||
|
|
resources, both will succeed (lock). If devices are in different buses and
|
||
|
|
trying to lock different resources, only the first who tried succeeds. ::
|
||
|
|
|
||
|
|
int pci_device_vgaarb_lock (void);
|
||
|
|
int pci_device_vgaarb_trylock (void);
|
||
|
|
|
||
|
|
Unlock resources of device. ::
|
||
|
|
|
||
|
|
int pci_device_vgaarb_unlock (void);
|
||
|
|
|
||
|
|
Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA
|
||
|
|
Memory, both, or none. All cards default to both, the card driver (fbdev for
|
||
|
|
example) should tell the arbiter if it has disabled legacy decoding, so the
|
||
|
|
card can be left out of the arbitration process (and can be safe to take
|
||
|
|
interrupts at any time. ::
|
||
|
|
|
||
|
|
int pci_device_vgaarb_decodes (int new_vgaarb_rsrc);
|
||
|
|
|
||
|
|
Connects to the arbiter device, allocates the struct ::
|
||
|
|
|
||
|
|
int pci_device_vgaarb_init (void);
|
||
|
|
|
||
|
|
Close the connection ::
|
||
|
|
|
||
|
|
void pci_device_vgaarb_fini (void);
|
||
|
|
|
||
|
|
xf86VGAArbiter (X server implementation)
|
||
|
|
----------------------------------------
|
||
|
|
|
||
|
|
X server basically wraps all the functions that touch VGA registers somehow.
|
||
|
|
|
||
|
|
References
|
||
|
|
----------
|
||
|
|
|
||
|
|
Benjamin Herrenschmidt (IBM?) started this work when he discussed such design
|
||
|
|
with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and
|
||
|
|
Tiago Vignatti (both of C3SL/Federal University of Paraná) proceeded his work
|
||
|
|
enhancing the kernel code to adapt as a kernel module and also did the
|
||
|
|
implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave
|
||
|
|
Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree.
|
||
|
|
|
||
|
|
0) http://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347
|
||
|
|
1) http://lists.freedesktop.org/archives/xorg/2005-March/006663.html
|
||
|
|
2) http://lists.freedesktop.org/archives/xorg/2005-March/006745.html
|
||
|
|
3) http://lists.freedesktop.org/archives/xorg/2007-October/029507.html
|