Hi, On Wed, Apr 14, 2021 at 5:58 PM Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> wrote: > > Hi Doug, > > Thank you for the patch. > > 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? > > - 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@xxxxxxxxxxxxxxxx> Thanks! _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
