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

[Laurent Pinchart]

Hi Jyri,

Thank you for the patch.

On Tuesday 07 Feb 2017 16:41:20 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);
> +		return -EINVAL;

This needs to be validated during the atomic check phase to avoid failures at 
commit time that are much harder to handle properly.

> +	}
> +
>  	*core_clk = dispc.feat->calc_core_clk(pclk, in_width, in_height,
>  				out_width, out_height, mem_to_mem);
>  	return 0;

-- 
Regards,

Laurent Pinchart