Re: [PATCH] drm: panel: simple: Delay HPD checking on boe_nv133fhm_n61 for 15 ms

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Thu 16 Jul 13:21 PDT 2020, 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.
> 

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>

> Signed-off-by: Douglas Anderson <dianders@chromium.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.
> 
>  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
>