Re: [PATCH v2 06/14] drm/dp-mst: Use dc and drm helpers to compute timeslots

[Lyude Paul <lyude-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

Some nitpicks below

On Tue, 2019-08-20 at 15:11 -0400, David Francis wrote:
> We were using drm helpers to convert a timing into its
> bandwidth, its bandwidth into pbn, and its pbn into timeslots
> 
> These helpers
> -Did not take DSC timings into account
> -Used the link rate and lane count of the link's aux device,
>  which are not the same as the link's current cap
> -Did not take FEC into account (FEC reduces the PBN per timeslot)
> 
> For converting timing into PBN, add a new function
> drm_dp_calc_pbn_mode_dsc that handles the DSC case
> 
> For converting PBN into time slots, amdgpu doesn't use the
> 'correct' atomic method (drm_dp_atomic_find_vcpi_slots), so
> don't add a new helper to cover our approach. Use the same
> means of calculating pbn per time slot as the DSC code.
> 
> v2: Add drm helper for clock to pbn conversion
> 
> Cc: Jerry Zuo <Jerry.Zuo@amd.com>
> Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
> Signed-off-by: David Francis <David.Francis@amd.com>
> ---
>  .../amd/display/amdgpu_dm/amdgpu_dm_helpers.c | 18 +++++---
>  drivers/gpu/drm/drm_dp_mst_topology.c         | 41 +++++++++++++++++++
>  include/drm/drm_dp_mst_helper.h               |  2 +-
>  3 files changed, 54 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> index 5f2c315b18ba..dfa99e0d6e64 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> @@ -189,8 +189,8 @@ bool dm_helpers_dp_mst_write_payload_allocation_table(
>  	int slots = 0;
>  	bool ret;
>  	int clock;
> -	int bpp = 0;
>  	int pbn = 0;
> +	int pbn_per_timeslot, bpp = 0;
>  
>  	aconnector = (struct amdgpu_dm_connector *)stream->dm_stream_context;
>  
> @@ -208,7 +208,6 @@ bool dm_helpers_dp_mst_write_payload_allocation_table(
>  		clock = stream->timing.pix_clk_100hz / 10;
>  
>  		switch (stream->timing.display_color_depth) {
> -
>  		case COLOR_DEPTH_666:
>  			bpp = 6;
>  			break;
> @@ -234,11 +233,18 @@ bool dm_helpers_dp_mst_write_payload_allocation_table(
>  
>  		bpp = bpp * 3;
>  
> -		/* TODO need to know link rate */
> -
> -		pbn = drm_dp_calc_pbn_mode(clock, bpp);
> +#ifdef CONFIG_DRM_AMD_DC_DSC_SUPPORT
> +		if (stream->timing.flags.DSC)
> +			pbn = drm_dp_calc_pbn_mode_dsc(clock,
> +					stream-
> >timing.dsc_cfg.bits_per_pixel);
> +		else
> +#endif
> +			pbn = drm_dp_calc_pbn_mode(clock, bpp);
>  
> -		slots = drm_dp_find_vcpi_slots(mst_mgr, pbn);
> +		/* Convert kilobits per second / 64 (for 64 timeslots) to pbn
> (54/64 megabytes per second) */
> +		pbn_per_timeslot = dc_link_bandwidth_kbps(
> +				stream->link, dc_link_get_link_cap(stream-
> >link)) / (8 * 1000 * 54);
> +		slots = DIV_ROUND_UP(pbn, pbn_per_timeslot);
>  		ret = drm_dp_mst_allocate_vcpi(mst_mgr, mst_port, pbn, slots);
>  
>  		if (!ret)
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 398e7314ea8b..d789b7af7dbf 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -3588,6 +3588,47 @@ static int test_calc_pbn_mode(void)
>  	return 0;
>  }
>  
> +/**
> + * drm_dp_calc_pbn_mode_dsc() - Calculate the PBN for a mode with DSC
> enabled.
> + * @clock: dot clock for the mode
> + * @dsc_bpp: dsc bits per pixel x16 (e.g. dsc_bpp = 136 is 8.5 bpp)
> + *
> + * This uses the formula in the spec to calculate the PBN value for a mode,
> + * given that the mode is using DSC

You should use the proper kdoc formatting for documenting return values (not
all of the DP MST code does this currently, but that's a bug I haven't taken
the time to fix :):

/*
 * foo_bar() - foo the bar
 *
 * foos the bar, guaranteed
 * Returns:
 * Some magic number
 */

> + */
> +int drm_dp_calc_pbn_mode_dsc(int clock, int dsc_bpp)
> +{
> +	u64 kbps;
> +	s64 peak_kbps;
> +	u32 numerator;
> +	u32 denominator;
> +
> +	kbps = clock * dsc_bpp;
> +
> +	/*
> +	 * margin 5300ppm + 300ppm ~ 0.6% as per spec, factor is 1.006
> +	 * The unit of 54/64Mbytes/sec is an arbitrary unit chosen based on
> +	 * common multiplier to render an integer PBN for all link rate/lane
> +	 * counts combinations
> +	 * calculate
> +	 * peak_kbps *= (1/16) bppx16 to bpp
> +	 * peak_kbps *= (1006/1000)
> +	 * peak_kbps *= (64/54)
> +	 * peak_kbps *= 8    convert to bytes
> +	 *
> +	 * Divide numerator and denominator by 16 to avoid overflow
> +	 */
> +
> +	numerator = 64 * 1006 / 16;
> +	denominator = 54 * 8 * 1000 * 1000;
> +
> +	kbps *= numerator;
> +	peak_kbps = drm_fixp_from_fraction(kbps, denominator);
> +
> +	return drm_fixp2int_ceil(peak_kbps);
> +}
> +EXPORT_SYMBOL(drm_dp_calc_pbn_mode_dsc);
> +
>  /* we want to kick the TX after we've ack the up/down IRQs. */
>  static void drm_dp_mst_kick_tx(struct drm_dp_mst_topology_mgr *mgr)
>  {
> diff --git a/include/drm/drm_dp_mst_helper.h
> b/include/drm/drm_dp_mst_helper.h
> index 2ba6253ea6d3..ddb518f2157a 100644
> --- a/include/drm/drm_dp_mst_helper.h
> +++ b/include/drm/drm_dp_mst_helper.h
> @@ -611,7 +611,7 @@ struct edid *drm_dp_mst_get_edid(struct drm_connector
> *connector, struct drm_dp_
>  
>  
>  int drm_dp_calc_pbn_mode(int clock, int bpp);
> -
> +int drm_dp_calc_pbn_mode_dsc(int clock, int dsc_bpp);
>  
>  bool drm_dp_mst_allocate_vcpi(struct drm_dp_mst_topology_mgr *mgr,
>  			      struct drm_dp_mst_port *port, int pbn, int
> slots);
-- 
Cheers,
	Lyude Paul

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