[PATCH] drm/omapdrm: dispc: Refuse x-decimation above 4 for all but 8-bit formats

[Laurent Pinchart]

Hi Tomi,

On Wednesday 08 Feb 2017 15:51:08 Tomi Valkeinen wrote:
> On 07/02/17 16:41, Jyri Sarha wrote:
> > Let's disable all scaling that requires horizontal decimation with
> > higher factor than 4, until we have better estimates of what we can
> > and can not do. However, 1 byte per pixel color format appear to work
> > Ok with all decimation factors.
> > 
> > When decimating horizontally by more that 4 the dss is not able to
> > fetch the data in burst mode. When this happens it is hard to tell if
> > there enough bandwidth. Despite what theory says this appears to be
> > true also for 16-bit color formats.
> > 
> > Signed-off-by: Jyri Sarha <jsarha at ti.com>
> > ---
> > 
> >  drivers/gpu/drm/omapdrm/dss/dispc.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/omapdrm/dss/dispc.c
> > b/drivers/gpu/drm/omapdrm/dss/dispc.c index 5554b72..61daef6 100644
> > --- a/drivers/gpu/drm/omapdrm/dss/dispc.c
> > +++ b/drivers/gpu/drm/omapdrm/dss/dispc.c
> > @@ -2506,6 +2506,25 @@ static int dispc_ovl_calc_scaling_44xx(unsigned
> > long pclk, unsigned long lclk,> 
> >  		return -EINVAL;
> >  	
> >  	}
> > 
> > +	if (*decim_x > 4 && color_mode_to_bpp(color_mode) > 8) {
> > +		/*
> > +		  Let's disable all scaling that requires horizontal
> > +		  decimation with higher factor than 4, until we have
> > +		  better estimates of what we can and can not
> > +		  do. However, 1 byte per pixel color format appear to
> > +		  work Ok with all decimation factors.
> > +
> > +		  When decimating horizontally by more that 4 the dss
> > +		  is not able to fetch the data in burst mode. When
> > +		  this happens it is hard to tell if there enough
> > +		  bandwidth. Despite what theory says this appears to
> > +		  be true also for 16-bit color formats.
> > +		*/
> > +		DSSERR("Not enough bandwidth (x-decimation factor %d > 4)",
> > +			*decim_x);
> 
> I think the error message could be improved. A normal user could hit
> this when setting up a plane, and I'm quite sure the above doesn't give
> any clue to the user what the issue is (too much downscaling).

Shouldn't the message be turned into a debug message ? Otherwise you'll give a 
way for users to flood the kernel log, which is never good.

-- 
Regards,

Laurent Pinchart