linux-kernel - Re: [PATCH 1/1] drm/panel: truly: Add additional delay after pulling down reset gpio

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Date:   Tue, 28 May 2019 10:43:20 +0200
From:   Marc Gonzalez <marc.w.gonzalez@...e.fr>
To:     Vivek Gautam <vivek.gautam@...eaurora.org>, airlied@...ux.ie,
        thierry.reding@...il.com, daniel@...ll.ch
Cc:     Rob Clark <robdclark@...il.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        MSM <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] drm/panel: truly: Add additional delay after pulling
 down reset gpio

On 27/05/2019 12:26, Vivek Gautam wrote:

> MTP SDM845 panel seems to need additional delay to bring panel
> to a workable state. Running modetest without this change displays
> blurry artifacts.
> 
> Signed-off-by: Vivek Gautam <vivek.gautam@...eaurora.org>
> ---
>  drivers/gpu/drm/panel/panel-truly-nt35597.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/gpu/drm/panel/panel-truly-nt35597.c b/drivers/gpu/drm/panel/panel-truly-nt35597.c
> index fc2a66c53db4..aa7153fd3be4 100644
> --- a/drivers/gpu/drm/panel/panel-truly-nt35597.c
> +++ b/drivers/gpu/drm/panel/panel-truly-nt35597.c
> @@ -280,6 +280,7 @@ static int truly_35597_power_on(struct truly_nt35597 *ctx)
>  	gpiod_set_value(ctx->reset_gpio, 1);
>  	usleep_range(10000, 20000);
>  	gpiod_set_value(ctx->reset_gpio, 0);
> +	usleep_range(10000, 20000);

I'm not sure usleep_range() makes sense with these values.

AFAIU, usleep_range() is typically used for sub-jiffy sleeps, and is based
on HRT to generate an interrupt.

Once we get into jiffy granularity, it seems to me msleep() is good enough.
IIUC, it would piggy-back on the jiffy timer interrupt.

In short, why not just use msleep(10); ?

Regards.