linux-kernel - Re: [PATCH 3/8] drm/vc4: Use drm_mode_vrefresh() in DSI fixup, in case vrefresh is 0.

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Date:   Thu, 29 Jun 2017 11:24:49 +0200
From:   Andrzej Hajda <a.hajda@...sung.com>
To:     Eric Anholt <eric@...olt.net>, dri-devel@...ts.freedesktop.org,
        Archit Taneja <architt@...eaurora.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] drm/vc4: Use drm_mode_vrefresh() in DSI fixup,
 in case vrefresh is 0.

On 27.06.2017 21:58, Eric Anholt wrote:
> I'm not sure what changed where I started getting vrefresh=0 from the
> mode to be fixed up.

It can be a case of low pixel_clock value, maybe it should be
investigated further, unless there is execution path with forgotten
mode->vrefresh =

drm_mode_vrefresh(mode)

>
> Signed-off-by: Eric Anholt <eric@...olt.net>
> ---
>  drivers/gpu/drm/vc4/vc4_dsi.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c
> index 629d372633e6..fca4d7fd677e 100644
> --- a/drivers/gpu/drm/vc4/vc4_dsi.c
> +++ b/drivers/gpu/drm/vc4/vc4_dsi.c
> @@ -866,7 +866,9 @@ static bool vc4_dsi_encoder_mode_fixup(struct drm_encoder *encoder,
>  	adjusted_mode->clock = pixel_clock_hz / 1000 + 1;
>  
>  	/* Given the new pixel clock, adjust HFP to keep vrefresh the same. */
> -	adjusted_mode->htotal = pixel_clock_hz / (mode->vrefresh * mode->vtotal);
> +	adjusted_mode->htotal = pixel_clock_hz / (drm_mode_vrefresh(mode) *
> +						  mode->vtotal);
> +

I am not sure but I guess division by zero is also possible here.
I do not know if you need to handle interlaced/dblscan/vscan modes, but
maybe it would be safer to calculate adjusted_htotal according to:

adjusted_mode->htotal = pixel_clock_hz * mode->htotal / (mode->clock * 1000)

or sth similar.


Regards
Andrzej

>  	adjusted_mode->hsync_end += adjusted_mode->htotal - mode->htotal;
>  	adjusted_mode->hsync_start += adjusted_mode->htotal - mode->htotal;
>