Re: [PATCH v3 12/12] drm/panel: panel-simple: Use runtime pm to avoid excessive unprepare / prepare

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Doug,

On Wed, Apr 14, 2021 at 06:22:57PM -0700, Doug Anderson wrote:
> On Wed, Apr 14, 2021 at 5:58 PM Laurent Pinchart wrote:
> > On Fri, Apr 02, 2021 at 03:28:46PM -0700, Douglas Anderson wrote:
> > > Unpreparing and re-preparing a panel can be a really heavy
> > > operation. Panels datasheets often specify something on the order of
> > > 500ms as the delay you should insert after turning off the panel
> > > before turning it on again. In addition, turning on a panel can have
> > > delays on the order of 100ms - 200ms before the panel will assert HPD
> > > (AKA "panel ready"). The above means that we should avoid turning a
> > > panel off if we're going to turn it on again shortly.
> > >
> > > The above becomes a problem when we want to read the EDID of a
> > > panel. The way that ordering works is that userspace wants to read the
> > > EDID of the panel _before_ fully enabling it so that it can set the
> > > initial mode correctly. However, we can't read the EDID until we power
> > > it up. This leads to code that does this dance (like
> > > ps8640_bridge_get_edid()):
> > >
> > > 1. When userspace requests EDID / the panel modes (through an ioctl),
> > >    we power on the panel just enough to read the EDID and then power
> > >    it off.
> > > 2. Userspace then turns the panel on.
> > >
> > > There's likely not much time between step #1 and #2 and so we want to
> > > avoid powering the panel off and on again between those two steps.
> > >
> > > Let's use Runtime PM to help us. We'll move the existing prepare() and
> > > unprepare() to be runtime resume() and runtime suspend(). Now when we
> > > want to prepare() or unprepare() we just increment or decrement the
> > > refcount. We'll default to a 1 second autosuspend delay which seems
> > > sane given the typical delays we see for panels.
> > >
> > > A few notes:
> > > - It seems the existing unprepare() and prepare() are defined to be
> > >   no-ops if called extra times. We'll preserve that behavior.
> >
> > The prepare and unprepare calls are supposed to be balanced, which
> > should allow us to drop this check. Do you have a reason to suspect that
> > it may not be the case ?
> 
> No, it was just code inspection. The old code definitely made an
> effort to make enable of an already enabled panel a no-op and disable
> of an already disabled panel a no-op. This is even before my
> (somewhat) recent patch to make things timing based, though I did
> touch the code.
> 
> Can I maybe suggest that getting rid of the extra check should be a
> separate patch after this one? Then if it breaks someone it's easy to
> just revert that one and we can still keep the runtime pm?

Sounds good to me.

> > > - This is a slight change in the ABI of simple panel. If something was
> > >   absolutely relying on the unprepare() to happen instantly that
> > >   simply won't be the case anymore. I'm not aware of anyone relying on
> > >   that behavior, but if there is someone then we'll need to figure out
> > >   how to enable (or disable) this new delayed behavior selectively.
> > > - In order for this to work we now have a hard dependency on
> > >   "PM". From memory this is a legit thing to assume these days and we
> > >   don't have to find some fallback to keep working if someone wants to
> > >   build their system without "PM".
> >
> > Sounds fine to me.
> >
> > The code looks good to me. Possibly with the prepared check removed,
> >
> > Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

-- 
Regards,

Laurent Pinchart