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

From: Boris Brezillon
Date: Fri Aug 04 2017 - 04:53:22 EST


On Tue, 18 Jul 2017 14:05:05 -0700
Eric Anholt <eric@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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;
>