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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Nov 13, 2017 at 03:36:58PM +0100, Maarten Lankhorst wrote:
> Op 13-11-17 om 14:36 schreef Ville Syrjala:
> > From: Ville Syrjälä <ville.syrjala@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@ffwll.ch>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Fixes: 94f050246b42 ("drm/i915: nonblocking commit")
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> This patch won't really fix it, you could still have a blocking modeset in parallel to a nonblocking one.

Hence the flush_workqueue().

> What would really be needed to fix those instances here?

Someone needs to review the entire modeset code and come up with a fix
for all the cases where we are frobbing some global state.

-- 
Ville Syrjälä
Intel OTC
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