This patch implements a simple function for matching DRM device FDs against the desired properties of a device that you are looking for.
The properties that are possible to look for are the elements of DrmVersion and DrmDevice.
The discussion that led to this series can be found here: https://lists.freedesktop.org/archives/mesa-dev/2018-January/183878.html
In the previous discussion we left off having settled on implementing this in mesa/src/loader/loader.c, which I initially did. But the code ended up being so generic that there's no reason for it not to live in libdrm, since it can be used for any compositor and mesa.
The implementer will still have to iterate through all of the devices available on the target, and see if they match. This additional functionality could be moved into libdrm at a later point if it turns out that all of the users do this in the same manner. If there is some variety, for example with selecting fallback devices if nothing matches maybe it is best left up to the user of libdrm.
The biggest problem with the approach as implemented, the way I see it, is how we match against the DrmVersion/DrmDevice of a given FD. Currently we use DrmVersion & DrmDevice as supplied by the caller to define what we are looking for. This is a little bit problematic, especially for DrmDevice, since all of the elements of the struct do not have enough bitspace to signal that we are uninterested in looking for that specific element. DrmDevice uses drmDevicesEqual() to do what amounts to a memcmp of the DrmDevice argument and the one of the FD. So looking for any device on any PCI bus with just the PCI vendor ID supplied isn't possible.
An alternative Daniel Stone suggested is adding enums for different properties and allowing the caller to supply a list of properties that are interesting and their values. In terms of long-term maintainership this might be less pleasant than the approach of the current implementation.
Robert Foss (1): xf86drm: Add drmHandleMatch func
xf86drm.h | 2 ++ xf86drmMode.c | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 71 insertions(+)