[PATCH] drm/bridge: parade-ps8640: Wait for HPD when doing an AUX transfer

[Doug Anderson]

Hi,

On Mon, Dec 25, 2023 at 1:08 AM Pin-yen Lin <treapking at chromium.org> wrote:
>
> Hi,
>
> On Fri, Dec 22, 2023 at 11:34 PM Doug Anderson <dianders at chromium.org> wrote:
> >
> > Hi,
> >
> > On Fri, Dec 22, 2023 at 2:29 AM Pin-yen Lin <treapking at chromium.org> wrote:
> > >
> > > Hi Douglas,
> > >
> > > On Fri, Dec 22, 2023 at 5:56 AM Douglas Anderson <dianders at chromium.org> wrote:
> > > >
> > > > Unlike what is claimed in commit f5aa7d46b0ee ("drm/bridge:
> > > > parade-ps8640: Provide wait_hpd_asserted() in struct drm_dp_aux"), if
> > > > someone manually tries to do an AUX transfer (like via `i2cdump ${bus}
> > > > 0x50 i`) while the panel is off we don't just get a simple transfer
> > > > error. Instead, the whole ps8640 gets thrown for a loop and goes into
> > > > a bad state.
> > > >
> > > > Let's put the function to wait for the HPD (and the magical 50 ms
> > > > after first reset) back in when we're doing an AUX transfer. This
> > > > shouldn't actually make things much slower (assuming the panel is on)
> > > > because we should immediately poll and see the HPD high. Mostly this
> > > > is just an extra i2c transfer to the bridge.
> > > >
> > > > Fixes: f5aa7d46b0ee ("drm/bridge: parade-ps8640: Provide wait_hpd_asserted() in struct drm_dp_aux")
> > > > Signed-off-by: Douglas Anderson <dianders at chromium.org>
> > > > ---
> > > >
> > > >  drivers/gpu/drm/bridge/parade-ps8640.c | 5 +++++
> > > >  1 file changed, 5 insertions(+)
> > > >
> > > > diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
> > > > index 541e4f5afc4c..fb5e9ae9ad81 100644
> > > > --- a/drivers/gpu/drm/bridge/parade-ps8640.c
> > > > +++ b/drivers/gpu/drm/bridge/parade-ps8640.c
> > > > @@ -346,6 +346,11 @@ static ssize_t ps8640_aux_transfer(struct drm_dp_aux *aux,
> > > >         int ret;
> > > >
> > > >         pm_runtime_get_sync(dev);
> > > > +       ret = _ps8640_wait_hpd_asserted(ps_bridge, 200 * 1000);
> > > > +       if (ret) {
> > > > +               pm_runtime_put_sync_suspend(dev);
> > > > +               return ret;
> > > > +       }
> > > >         ret = ps8640_aux_transfer_msg(aux, msg);
> > > >         pm_runtime_mark_last_busy(dev);
> > > >         pm_runtime_put_autosuspend(dev);
> > > > --
> > > > 2.43.0.472.g3155946c3a-goog
> > > >
> > >
> > > I think commit 9294914dd550 ("drm/bridge: parade-ps8640: Link device
> > > to ensure suspend/resume order")  is trying to address the same
> > > problem, but we see this issue here because the device link is missing
> > > DL_FLAG_PM_RUNTIME. I prefer to add DL_FLAG_PM_RUNTIME here so we
> > > don't need to add a _ps8640_wait_hpd_asserted() after every
> > > pm_runtime_get_*() call.
> >
> > I disagree. We've had several discussions on the lists about this
> > topic before, though since I'm technically on vacation right now I'm
> > not going to go look them up. In general "pm_runtime" is not
> > sufficient for powering up DRM components. While DRM components can
> > use pm_runtime themselves (as we are doing here), powering up another
> > DRM component by grabbing a pm_runtime reference isn't right. There is
> > a reason for the complexity of the DRM prepare/enable and all the
> > current debates about the right order to call components in prepare()
> > just demonstrates further that a simple pm_runtime reference isn't
> > enough.
> >
> > It can be noted that, with ${SUBJECT} patch we _aren't_ powering up
> > the panel. I actually tested two cases on sc7180-lazor. In one case I
> > just closed the lid, which powered off the panel, but the touchscreen
> > kept the panel power rail on. In this case with my patch I could still
> > read the panel EDID. I then hacked the touchscreen off. Now when I
> > closed the lid I'd get a timeout. This is different than if we tried
> > to get a pm_runtime reference to the panel.
> >
> Okay, thanks for the detailed explanation. Then, let's go with the
> approach in this patch. So,
>
> Tested-by: Pin-yen Lin <treapking at chromium.org>
> Reviewed-by: Pin-yen Lin <treapking at chromium.org>

Thanks for the tags. I've pushed this to drm-misc-fixes:

024b32db43a3 drm/bridge: parade-ps8640: Wait for HPD when doing an AUX transfer