linux-kernel - Re: [PATCH] drm/msm/dsi: Implement reset correctly

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Date:   Thu, 10 Oct 2019 14:45:44 -0400
From:   Sean Paul <sean@...rly.run>
To:     Jeffrey Hugo <jeffrey.l.hugo@...il.com>
Cc:     robdclark@...il.com, sean@...rly.run, airlied@...ux.ie,
        daniel@...ll.ch, linux-arm-msm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/msm/dsi: Implement reset correctly

On Wed, Oct 09, 2019 at 02:34:54PM -0700, Jeffrey Hugo wrote:
> On msm8998, vblank timeouts are observed because the DSI controller is not
> reset properly, which ends up stalling the MDP.  This is because the reset
> logic is not correct per the hardware documentation.
> 
> The documentation states that after asserting reset, software should wait
> some time (no indication of how long), or poll the status register until it
> returns 0 before deasserting reset.
> 
> wmb() is insufficient for this purpose since it just ensures ordering, not
> timing between writes.  Since asserting and deasserting reset occurs on the
> same register, ordering is already guaranteed by the architecture, making
> the wmb extraneous.
> 
> Since we would define a timeout for polling the status register to avoid a
> possible infinite loop, lets just use a static delay of 20 ms, since 16.666
> ms is the time available to process one frame at 60 fps.
> 
> Fixes: a689554ba6ed (drm/msm: Initial add DSI connector support)
> Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo@...il.com>
> ---
> 
> Rob et al, is it possible for this to go into a 5.4-rc?
> 
>  drivers/gpu/drm/msm/dsi/dsi_host.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
> index 663ff9f4fac9..68ded9b4735d 100644
> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> @@ -986,7 +986,7 @@ static void dsi_sw_reset(struct msm_dsi_host *msm_host)
>  	wmb(); /* clocks need to be enabled before reset */
>  
>  	dsi_write(msm_host, REG_DSI_RESET, 1);
> -	wmb(); /* make sure reset happen */
> +	msleep(20); /* make sure reset happen */

Could you please pull this out into a #define used for both in case we decide to
tweak it? I don't want these 2 values to drift.

Thanks,
Sean

>  	dsi_write(msm_host, REG_DSI_RESET, 0);
>  }
>  
> @@ -1396,7 +1396,7 @@ static void dsi_sw_reset_restore(struct msm_dsi_host *msm_host)
>  
>  	/* dsi controller can only be reset while clocks are running */
>  	dsi_write(msm_host, REG_DSI_RESET, 1);
> -	wmb();	/* make sure reset happen */
> +	msleep(20);	/* make sure reset happen */
>  	dsi_write(msm_host, REG_DSI_RESET, 0);
>  	wmb();	/* controller out of reset */
>  	dsi_write(msm_host, REG_DSI_CTRL, data0);
> -- 
> 2.17.1
> 

-- 
Sean Paul, Software Engineer, Google / Chromium OS