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

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

Le Fri, 04 Aug 2017 14:15:56 -0700,
Eric Anholt <eric@anholt.net> a écrit :

> Boris Brezillon <boris.brezillon@free-electrons.com> writes:
> 
> > On Tue, 18 Jul 2017 14:05:05 -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.
> >> 
> >> Signed-off-by: Eric Anholt <eric@anholt.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?  
> 
> We're actually expecting to get within epsilon of pixel_clock_hz, but we
> have to bump our clk_set_rate() to a higher value because the clock
> driver will give you a bad divider if you ask for anything less than the
> rate it can provide.
> 
> How about I don't increment the adjusted_mode->clock (since it'll be
> userspace visible I think), and instead move that and the "Round up"
> comment to the clk_set_rate()?

Sounds good.