[PATCH v2 09/22] drm/dp_mst: Fix fractional bpp scaling in drm_dp_calc_pbn_mode()

[Lyude Paul]

Amazing! This work looks awesome Imre, sorry it took me a little bit to get
back to this :). For all of the DP MST helper patches:

Reviewed-by: Lyude Paul <lyude at redhat.com>

On Thu, 2023-08-24 at 11:05 +0300, Imre Deak wrote:
> For fractional bpp values passed to the function in a .4 fixed point
> format, the fractional part is currently ignored due to scaling bpp too
> early. Fix this by scaling the overhead factor instead and to avoid an
> overflow multiplying bpp with the overhead factor instead of the clock
> rate.
> 
> While at it simplify the formula, and pass the expected fixed point bpp
> values in the kunit tests.
> 
> Cc: Lyude Paul <lyude at redhat.com>
> Cc: dri-devel at lists.freedesktop.org
> Signed-off-by: Imre Deak <imre.deak at intel.com>
> ---
>  drivers/gpu/drm/display/drm_dp_mst_topology.c  | 7 ++-----
>  drivers/gpu/drm/tests/drm_dp_mst_helper_test.c | 8 ++++----
>  2 files changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index ed96cfcfa3040..bd0f35a0ea5fb 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -4712,12 +4712,9 @@ int drm_dp_calc_pbn_mode(int clock, int bpp, bool dsc)
>  	 * factor in the numerator rather than the denominator to avoid
>  	 * integer overflow
>  	 */
> +	u32 bpp_m = (dsc ? 64 / 16 : 64) * 1006 * bpp;
>  
> -	if (dsc)
> -		return DIV_ROUND_UP_ULL(mul_u32_u32(clock * (bpp / 16), 64 * 1006),
> -					8 * 54 * 1000 * 1000);
> -
> -	return DIV_ROUND_UP_ULL(mul_u32_u32(clock * bpp, 64 * 1006),
> +	return DIV_ROUND_UP_ULL(mul_u32_u32(clock, bpp_m),
>  				8 * 54 * 1000 * 1000);
>  }
>  EXPORT_SYMBOL(drm_dp_calc_pbn_mode);
> diff --git a/drivers/gpu/drm/tests/drm_dp_mst_helper_test.c b/drivers/gpu/drm/tests/drm_dp_mst_helper_test.c
> index 545beea33e8c7..ea2182815ebe8 100644
> --- a/drivers/gpu/drm/tests/drm_dp_mst_helper_test.c
> +++ b/drivers/gpu/drm/tests/drm_dp_mst_helper_test.c
> @@ -40,15 +40,15 @@ static const struct drm_dp_mst_calc_pbn_mode_test drm_dp_mst_calc_pbn_mode_cases
>  	},
>  	{
>  		.clock = 332880,
> -		.bpp = 24,
> +		.bpp = 24 << 4,
>  		.dsc = true,
> -		.expected = 50
> +		.expected = 1191
>  	},
>  	{
>  		.clock = 324540,
> -		.bpp = 24,
> +		.bpp = 24 << 4,
>  		.dsc = true,
> -		.expected = 49
> +		.expected = 1161
>  	},
>  };
>  

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat