linux-kernel - Re: [PATCH v5 1/6] drm/vc4: Avoid using vrefresh==0 mode in DSI htotal math.

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Date:   Fri, 4 Aug 2017 10:53:04 +0200
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Eric Anholt <eric@...olt.net>
Cc:     dri-devel@...ts.freedesktop.org,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/6] drm/vc4: Avoid using vrefresh==0 mode in DSI
 htotal math.

On Tue, 18 Jul 2017 14:05:05 -0700
Eric Anholt <eric@...olt.net> wrote:

> The incoming mode might have a missing vrefresh field if it came from
> drmModeSetCrtc(), which the kernel is supposed to calculate using
> drm_mode_vrefresh().  We could either use that or the adjusted_mode's
> original vrefresh value.
> 
> However, we can maintain a more exact vrefresh value (not just the
> integer approximation), by scaling by the ratio of our clocks.
> 
> v2: Use math suggested by Andrzej Hajda instead.
> 
> Signed-off-by: Eric Anholt <eric@...olt.net>
> ---
>  drivers/gpu/drm/vc4/vc4_dsi.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c
> index 629d372633e6..57213f4e3c72 100644
> --- a/drivers/gpu/drm/vc4/vc4_dsi.c
> +++ b/drivers/gpu/drm/vc4/vc4_dsi.c
> @@ -866,7 +866,8 @@ 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 / 1000 * mode->htotal /
> +				 mode->clock);

Hm, I'm not sure I understand this. Shouldn't we have something like:

	adjusted_mode->htotal = (adjusted_mode->clock * mode->htotal) /
				mode->clock;

Is there a reason for doing '+ 1' when you calculate the adjusted
pixel clock rate but not here?

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