[Intel-gfx] [PATCH] drm/i915: Put all non-blocking modesets onto an ordered wq

[Daniel Vetter]

On Mon, Nov 13, 2017 at 3:36 PM, Maarten Lankhorst
<maarten.lankhorst at linux.intel.com> wrote:
> Op 13-11-17 om 14:36 schreef Ville Syrjala:
>> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
>>
>> We have plenty of global registers and whatnot programmed without
>> any further locking by the modeset code. Currently non-bocking
>> modesets are allowed to execute in parallel which could corrupt
>> said registers.
>>
>> To avoid the problem let's run all non-blocking modesets on an
>> ordered workqueue. We still put page flips etc. to system_unbound_wq
>> allowing page flips on one pipe to execute in parallel with page flips
>> or a modeset on a another pipe (assuming no known state is shared
>> between them, at which point they would have been added to the same
>> atomic commit and serialized that way).
>>
>> Blocking modesets are already serialized with each other by
>> connection_mutex, and thus are safe. To serialize them with
>> non-blocking modesets we just flush the workqueue before executing
>> blocking modesets.
>>
>> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
>> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
>> Fixes: 94f050246b42 ("drm/i915: nonblocking commit")
>> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> This patch won't really fix it, you could still have a blocking modeset in parallel to a nonblocking one. What would really be needed to fix those instances here?

The idea was that anything touching anything global would grap all
crtc state, and then we'd track those using the drm_crtc_commit stuff.
Putting everything onto one queue also doesn't work because they're
meant to somewhat overlap (plane cleanup is in the same work, but
should/can overlap with the next update).

Imo the right fix is to make sure we do add all the crtc states
everywhere we touch something global. And if that doesn't scale, then
modeset objects to track those bits.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch