On Thu, Jul 16, 2020 at 01:21:22PM -0700, Douglas Anderson 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@xxxxxxxxxxxx>
Nice drawing and good explanation.
Applied to drm-misc-fixes.
Sam
> ---
> 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.
>
> drivers/gpu/drm/panel/panel-simple.c | 16 +++++++++++++++-
> 1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> index 88493538a147..046a06b55800 100644
> --- a/drivers/gpu/drm/panel/panel-simple.c
> +++ b/drivers/gpu/drm/panel/panel-simple.c
> @@ -1260,7 +1260,21 @@ static const struct panel_desc boe_nv133fhm_n61 = {
> .height = 165,
> },
> .delay = {
> - .hpd_absent_delay = 200,
> + /*
> + * When power is first given to the panel there's a short
> + * spike on the HPD line. It was explained that this spike
> + * was until the TCON data download was complete. On
> + * one system this was measured at 8 ms. We'll put 15 ms
> + * in the prepare delay just to be safe and take it away
> + * from the hpd_absent_delay (which would otherwise be 200 ms)
> + * to handle this. That means:
> + * - If HPD isn't hooked up you still have 200 ms delay.
> + * - If HPD is hooked up we won't try to look at it for the
> + * first 15 ms.
> + */
> + .prepare = 15,
> + .hpd_absent_delay = 185,
> +
> .unprepare = 500,
> },
> .bus_format = MEDIA_BUS_FMT_RGB888_1X24,
> --
> 2.28.0.rc0.105.gf9edc3c819-goog
>
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