On Thu, 18 Apr 2013, David Herrmann wrote:
Hi
On Wed, Apr 17, 2013 at 11:05 PM, Byron Stanoszek gandalf@winds.org wrote:
David,
I'm developing a small application that uses libdrm (DRM ioctls) to change the resolution of a single graphics display and show a framebuffer. I've run into two problems with this implementation that I'm hoping you can address.
- Each application is its own process, which is designed to control 1
graphics display. This is unlike X, for instance, which could be configured to grab all of the displays in the system at once.
Depending on our stackup, there can be as many as 4 displays connected to a single graphics card. One process could open /dev/dri/card0 and call drmModeSetCrtc() to initialize one of its displays to the requested resolution. However, whenever a second process calls drmModeSetCrtc() to control a second display on the same card, it gets -EPERM back from the ioctl.
I've traced this down to the following line in linux/drivers/gpu/drm/drm_drv.c:
DRM_IOCTL_DEF(DRM_IOCTL_MODE_SETCRTC, drm_mode_setcrtc, DRM_MASTER|DRM_CONTROL_ALLOW|DRM_UNLOCKED),
If I remove the DRM_MASTER flag, then my application behaves correctly, and 4 separate processes can then control each individual display on the card without issue.
My question is, is there any real benefit to restricting drm_mode_setcrtc() with DRM_MASTER, or can we lose this flag in order to support one-process-per- display programs like the above?
Only one open-file can be DRM-Master. And only DRM-Master is allowed to perform mode-setting. This is to prevent render-clients (like OpenGL clients) to perform mode-setting, which should be restricted to the compositor/...
In your scenario, you should share a single open-file between the processes by passing the FDs to each. Or do all of that in a single process. There is no way to split CRTCs/connectors between different nodes or have multiple DRM-Masters on a single node at once. (There is work going on to allow this, but it will take a while...)
If running a custom-patched kernel is acceptable (i.e. custom-built, embedded system or the like), then a set of patches that I sent about a year ago [1] will probably do the job. The problem is that these patches are apparently not going upstream because there was little interest and there were a couple of arguments against them [2]. Originally, it's the work that Dave Airlie started but abandoned. I finished it off and tried to have it included upstream but it didn't happen.
I have an application that is similar to what is described here and I am using these patches to make it work. Essentially, what you do is call a small userspace utility (also included with the patches for libdrm) and specify which CRTCs/encoders/connectors/planes do you want included in the node and you get a new /dev/dri/render<N> node that your application can use and be the master on these resources only. Then for the next node you call the utility again and specify a new set of display resources and then you run the other application on the top of that node.
The patches that are on the mailing list archive [1] are now a year old, but I have a rebased version for newer kernels, which I can send to whoever is interested in having them (I am just hesitating to pollute the mailing list with patches to which the maintainers have already said "no").
[1] http://lists.freedesktop.org/archives/dri-devel/2012-April/021326.html [2] http://lists.freedesktop.org/archives/dri-devel/2012-September/028348.html
-- Ilija