Re: [PATCH v6 2/5] drm/vc4: Avoid using vrefresh==0 mode in DSI htotal math.

[Boris Brezillon <boris.brezillon@free-electrons.com>]

On Tue, 15 Aug 2017 16:47:19 -0700
Eric Anholt <eric@anholt.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.
> v3: Simplify math now that adjusted_mode->clock isn't padded.
> 
> Signed-off-by: Eric Anholt <eric@anholt.net>

Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>

> ---
>  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 eb787eed8abe..2516cd3a1d87 100644
> --- a/drivers/gpu/drm/vc4/vc4_dsi.c
> +++ b/drivers/gpu/drm/vc4/vc4_dsi.c
> @@ -862,7 +862,8 @@ static bool vc4_dsi_encoder_mode_fixup(struct drm_encoder *encoder,
>  	adjusted_mode->clock = pixel_clock_hz / 1000;
>  
>  	/* 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 = (adjusted_mode->clock * mode->htotal /
> +				 mode->clock);
>  	adjusted_mode->hsync_end += adjusted_mode->htotal - mode->htotal;
>  	adjusted_mode->hsync_start += adjusted_mode->htotal - mode->htotal;
>