Re: [PATCH v6 1/8] drm/msm/dp: Add eDP support via aux_bus

[Doug Anderson <dianders@chromium.org>]

Hi,

On Thu, Apr 7, 2022 at 7:19 AM Sankeerth Billakanti (QUIC)
<quic_sbillaka@quicinc.com> wrote:
>
> Hi Dmitry and Doug,
>
> > Hi,
> >
> > On Tue, Apr 5, 2022 at 10:36 AM Dmitry Baryshkov
> > <dmitry.baryshkov@linaro.org> wrote:
> > >
> > > On 05/04/2022 20:02, Doug Anderson wrote:
> > > > Hi,
> > > >
> > > > On Tue, Apr 5, 2022 at 5:54 AM Dmitry Baryshkov
> > > > <dmitry.baryshkov@linaro.org> wrote:
> > > >>> 3. For DP and eDP HPD means something a little different.
> > > >>> Essentially there are two concepts: a) is a display physically
> > > >>> connected and b) is the display powered up and ready. For DP, the
> > > >>> two are really tied together. From the kernel's point of view you
> > > >>> never "power down" a DP display and you can't detect that it's
> > > >>> physically connected until it's ready. Said another way, on you
> > > >>> tie "is a display there" to the HPD line and the moment a display
> > > >>> is there it's ready for you to do AUX transfers. For eDP, in the
> > > >>> lowest power state of a display it _won't_ assert its "HPD"
> > > >>> signal. However, it's still physically present. For eDP you simply
> > > >>> have to _assume_ it's present without any actual proof since you
> > > >>> can't get proof until you power it up. Thus for eDP, you report
> > > >>> that the display is there as soon as we're asked. We can't _talk_
> > > >>> to the display yet, though. So in get_modes() we need to be able
> > > >>> to power the display on enough to talk over the AUX channel to it.
> > > >>> As part of this, we wait for the signal named "HPD" which really means
> > "panel finished powering on" in this context.
> > > >>>
> > > >>> NOTE: for aux transfer, we don't have the _display_ pipe and
> > > >>> clocks running. We only have enough stuff running to do the AUX
> > transfer.
> > > >>> We're not clocking out pixels. We haven't fully powered on the
> > > >>> display. The AUX transfer is designed to be something that can be
> > > >>> done early _before_ you turn on the display.
> > > >>>
> > > >>>
> > > >>> OK, so basically that was a longwinded way of saying: yes, we
> > > >>> could avoid the AUX transfer in probe, but we can't wait all the
> > > >>> way to enable. We have to be able to transfer in get_modes(). If
> > > >>> you think that's helpful I think it'd be a pretty easy patch to
> > > >>> write even if it would look a tad bit awkward IMO. Let me know if
> > > >>> you want me to post it up.
> > > >>
> > > >> I think it would be a good idea. At least it will allow us to
> > > >> judge, which is the more correct way.
> > > >
> > > > I'm still happy to prototype this, but the more I think about it the
> > > > more it feels like a workaround for the Qualcomm driver. The eDP
> > > > panel driver is actually given a pointer to the AUX bus at probe
> > > > time. It's really weird to say that we can't do a transfer on it
> > > > yet... As you said, this is a little sideband bus. It should be able
> > > > to be used without all the full blown infra of the rest of the driver.
> > >
> > > Yes, I have that feeling too. However I also have a feeling that just
> > > powering up the PHY before the bus probe is ... a hack. There are no
> > > obvious stopgaps for the driver not to power it down later.
> >
> > This is why I think we need to move to Runtime PM to manage this. Basically:
> >
> > 1. When an AUX transfer happens, you grab a PM runtime reference that
> > _that_ powers up the PHY.
> >
> > 2. At the end of the AUX transfer function, you do a "put_autosuspend".
> >
> > Then it becomes not a hack, right?
> >
> >
>
> pm runtime ops needs to be implemented for both eDP and DP. This change
> take good amount of planning and code changes as it affects DP also.
>
> Because this patch series consist of basic eDP changes for SC7280 bootup,
> shall we take this pm_runtime implementation in subsequent patch series?

Dmitry is the real decision maker here, but in my opinion it would be
OK to get something landed first that worked OK and wasn't taking us
too far in the wrong direction and then we could get a follow up patch
to move to pm_runtime.