Re: [PATCH 1/4] drm/atomic: integrate modeset lock with private objects

[Ville Syrjälä]

On Wed, Feb 21, 2018 at 11:17:21AM -0500, Rob Clark wrote:
> On Wed, Feb 21, 2018 at 10:54 AM, Ville Syrjälä
> <ville.syrjala@xxxxxxxxxxxxxxx> wrote:
> > On Wed, Feb 21, 2018 at 10:36:06AM -0500, Rob Clark wrote:
> >> On Wed, Feb 21, 2018 at 10:27 AM, Ville Syrjälä
> >> <ville.syrjala@xxxxxxxxxxxxxxx> wrote:
> >> > On Wed, Feb 21, 2018 at 10:20:03AM -0500, Rob Clark wrote:
> >> >> On Wed, Feb 21, 2018 at 10:07 AM, Ville Syrjälä
> >> >> <ville.syrjala@xxxxxxxxxxxxxxx> wrote:
> >> >> > On Wed, Feb 21, 2018 at 09:54:49AM -0500, Rob Clark wrote:
> >> >> >> On Wed, Feb 21, 2018 at 9:49 AM, Ville Syrjälä
> >> >> >> <ville.syrjala@xxxxxxxxxxxxxxx> wrote:
> >> >> >> > On Wed, Feb 21, 2018 at 09:37:21AM -0500, Rob Clark wrote:
> >> >> >> >> Follow the same pattern of locking as with other state objects.  This
> >> >> >> >> avoids boilerplate in the driver.
> >> >> >> >
> >> >> >> > I'm not sure we really want to do this. What if the driver wants a
> >> >> >> > custom locking scheme for this state?
> >> >> >>
> >> >> >> That seems like something we want to discourage, ie. all the more
> >> >> >> reason for this patch.
> >> >> >>
> >> >> >> There is no reason drivers could not split their global state into
> >> >> >> multiple private objs's, each with their own lock, for more fine
> >> >> >> grained locking.  That is basically the only valid reason I can think
> >> >> >> of for "custom locking".
> >> >> >
> >> >> > In i915 we have at least one case that would want something close to an
> >> >> > rwlock. Any crtc lock is enough for read, need all of them for write.
> >> >> > Though if we wanted to use private objs for that we might need to
> >> >> > actually make the states refcounted as well, otherwise I can imagine
> >> >> > we might land in some use-after-free issues once again.
> >> >> >
> >> >> > Maybe we could duplicate the state into per-crtc and global copies, but
> >> >> > then we have to keep all of those in sync somehow which doesn't sound
> >> >> > particularly pleasant.
> >> >>
> >> >> Or just keep your own driver lock for read, and use that plus the core
> >> >> modeset lock for write?
> >> >
> >> > If we can't add the private obj to the state we can't really use it.
> >> >
> >>
> >> I'm not sure why that is strictly true (that you need to add it to the
> >> state if for read-only), since you'd be guarding it with your own
> >> driver read-lock you can just priv->foo_state->bar.
> >>
> >> Since it is read-only access, there is no roll-back to worry about for
> >> test-only or failed atomic_check()s..
> >
> > That would be super ugly. We want to access the information the same
> > way whether it has been modified or not.
> 
> Well, I mean the whole idea of what you want to do seems a bit super-ugly ;-)
> 
> I mean, in mdp5 the assigned global resources go in plane/crtc state,
> and tracking of what is assigned to which plane/crtc is in global
> state, so it fits nicely in the current locking model.  For i915, I'm
> not quite sure what is the global state you are concerned about, so it
> is a bit hard to talk about the best solution in the abstract.  Maybe
> the better option is to teach modeset-lock how to be a rwlock instead?

The thing I'm thinking is the core display clock (cdclk) frequency which
we need to consult whenever computing plane states and whatnot. We don't
want a modeset on one crtc to block a plane update on another crtc
unless we actually have to bump the cdclk (which would generally require
all crtcs to undergo a full modeset). Seems like a generally useful
pattern to me.

-- 
Ville Syrjälä
Intel OTC