- it's what we recommend in our docs:

https://dri.freedesktop.org/docs/drm/gpu/drm-uapi.html#recommended-ioctl-return-values

- it's the overwhelmingly used error code for "operation not
  supported", at least in drm core (slightly less so in drivers):

$ git grep EOPNOTSUPP -- drivers/gpu/drm/*c | wc -l
83
$ git grep ENOTSUPP -- drivers/gpu/drm/*c | wc -l
5

- include/linux/errno.h makes it fairly clear that these are for nfsv3
  (plus they also have error codes above 512, which is the block with
  some special behaviour ...)

/* Defined for the NFSv3 protocol */

If the above isn't reflecting current practice, then I guess we should
at least update the docs.

Noralf commented:

Ben Hutchings made this comment[1] in a thread about use of ENOTSUPP in
drivers:

  glibc's strerror() returns these strings for ENOTSUPP and EOPNOTSUPP
  respectively:

  "Unknown error 524"
  "Operation not supported"

So at least for errors returned to userspace EOPNOTSUPP makes sense.

José asked:

> Hopefully this will not break any userspace

None of the functions in drm_edid.c affected by this reach userspace,
it's all driver internal.

Same for the mipi function, that error code should be handled by
drivers. Drivers are supposed to remap "the hw is on fire" to EIO when
reporting up to userspace, but I think if a driver sees this it would
be a driver bug.
v2: Augment commit message with comments from Noralf and José
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Sean Paul <sean@poorly.run>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Andres Rodriguez <andresx7@gmail.com>
Cc: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190904143942.31756-1-daniel.vetter@ffwll.ch