DRM

It's a commond api for doing graphics on linux.

Some parts of it are intentionally driver specific.

Files / clients

Opening /dev/dri/card%d gives a unique DRM_MINOR_PRIMARY client. Opening /dev/dri/renderD%d gives a unique DRM_MINOR_RENDER client. Opening /dev/dri/accel%d gives a unique DRM_MINOR_ACCEL client.

Each gpu should get a primary and render files.

You'll most likely want to use the RENDER client.

If you need to have multiple file descriptors to a drm file simply dupplicate them with dup().

Permission structure

drm_ioctl_permit() is used to determine if the user have sufficient permisions to invoke an IOCTL.

These are the relevand flags set for IOCTLs:

• DRM_ROOT_ONLY - only allow when capable(CAP_SYS_ADMIN), effectively deprecated
• DRM_AUTH - only allow authenticated primary clients.
• DRM_MASTER - only allow current master
• DRM_RENDER_ALLOW - unless set, render clients not allowed

You can see currently existing drm_file and info if they are master or authenticated in corresponding drm debugfs /sys/kernel/debug/dri/*/clients.

MASTER

There can be at most only one, set for a device, at a time. You might get master status by opening a primary client or using SET_MASTER ioctl on a primary client after the previous master closed or used DROP_MASTER ioctl.

Reference counted

Opening these files return a reference counted object for this process, which means opening the files multiple times or dupplicating these file descriptors still reference the same object.

Message format

Commands here use PM4 format.

Source

More info in kernel/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c.