linux-kernel - Re: [PATCH] drm: panel: simple: Delay HPD checking on boe_nv133fhm

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Date:   Mon, 20 Jul 2020 13:24:00 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Thierry Reding <thierry.reding@...il.com>,
        Sam Ravnborg <sam@...nborg.org>
Cc:     Rob Clark <robdclark@...omium.org>,
        Steev Klimaszewski <steev@...i.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm: panel: simple: Delay HPD checking on
 boe_nv133fhm_n61 for 15 ms

Hi,


On Thu, Jul 16, 2020 at 1:21 PM Douglas Anderson <dianders@...omium.org> wrote:
>
> On boe_nv133fhm_n62 (and presumably on boe_nv133fhm_n61) a scope shows
> a small spike on the HPD line right when you power the panel on.  The
> picture looks something like this:
>
>          +--------------------------------------
>          |
>          |
>          |
> Power ---+
>                                            +---
>                                            |
>               ++                           |
>          +----+|                           |
> HPD -----+     +---------------------------+
>
> So right when power is applied there's a little bump in HPD and then
> there's small spike right before it goes low.  The total time of the
> little bump plus the spike was measured on one panel as being 8 ms
> long.  The total time for the HPD to go high on the same panel was
> 51.2 ms, though the datasheet only promises it is < 200 ms.
>
> When asked about this glitch, BOE indicated that it was expected and
> persisted until the TCON has been initialized.
>
> If this was a real hotpluggable DP panel then this wouldn't matter a
> whole lot.  We'd debounce the HPD signal for a really long time and so
> the little blip wouldn't hurt.  However, this is not a hotpluggable DP
> panel and the the debouncing logic isn't needed and just shows down
> the time needed to get the display working.  This is why the code in
> panel_simple_prepare() doesn't do debouncing and just waits for HPD to
> go high once.  Unfortunately if we get unlucky and happen to poll the
> HPD line right at the spike we can try talking to the panel before
> it's ready.
>
> Let's handle this situation by putting in a 15 ms prepare delay and
> decreasing the "hpd absent delay" by 15 ms.  That means:
> * If you don't have HPD hooked up at all you've still got the
>   hardcoded 200 ms delay.
> * If you've got HPD hooked up you will always wait at least 15 ms
>   before checking HPD.  The only case where this could be bad is if
>   the panel is sharing a voltage rail with something else in the
>   system and was already turned on long before the panel came up.  In
>   such a case we'll be delaying 15 ms for no reason, but it's not a
>   huge delay and I don't see any other good solution to handle that
>   case.
>
> Even though the delay was measured as 8 ms, 15 ms was chosen to give a
> bit of margin.
>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
> I don't actually have a device in front of me that is exhibiting these
> problems.  I believe that it is only some devices and some of the
> time.  Still, this patch seems safe and seems likely to fix the issue
> given the scope shots.

Just to follow-up, I just heard that someone who had a panel
exhibiting this problem tried my patch and it fixed it for them.  :-)
So this is not such a shot in the dark anymore.

-Doug