Re: [PATCH 01/19] drm/dp: Add drm_dp_max_dprx_data_rate()

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Jan 23, 2024 at 12:28:32PM +0200, Imre Deak wrote:
> Copy intel_dp_max_data_rate() to DRM core. It will be needed by a
> follow-up DP tunnel patch, checking the maximum rate the DPRX (sink)
> supports. Accordingly use the drm_dp_max_dprx_data_rate() name for
> clarity. This patchset will also switch calling the new DRM function
> in i915 instead of intel_dp_max_data_rate().
> 
> Signed-off-by: Imre Deak <imre.deak@intel.com>
> ---
>  drivers/gpu/drm/display/drm_dp_helper.c | 58 +++++++++++++++++++++++++
>  include/drm/display/drm_dp_helper.h     |  2 +
>  2 files changed, 60 insertions(+)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> index b1ca3a1100dab..24911243d4d3a 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -4058,3 +4058,61 @@ int drm_dp_bw_channel_coding_efficiency(bool is_uhbr)
>  		return 800000;
>  }
>  EXPORT_SYMBOL(drm_dp_bw_channel_coding_efficiency);
> +
> +/*
> + * Given a link rate and lanes, get the data bandwidth.
> + *
> + * Data bandwidth is the actual payload rate, which depends on the data
> + * bandwidth efficiency and the link rate.
> + *
> + * For 8b/10b channel encoding, SST and non-FEC, the data bandwidth efficiency
> + * is 80%. For example, for a 1.62 Gbps link, 1.62*10^9 bps * 0.80 * (1/8) =
> + * 162000 kBps. With 8-bit symbols, we have 162000 kHz symbol clock. Just by
> + * coincidence, the port clock in kHz matches the data bandwidth in kBps, and
> + * they equal the link bit rate in Gbps multiplied by 100000. (Note that this no
> + * longer holds for data bandwidth as soon as FEC or MST is taken into account!)
> + *
> + * For 128b/132b channel encoding, the data bandwidth efficiency is 96.71%. For
> + * example, for a 10 Gbps link, 10*10^9 bps * 0.9671 * (1/8) = 1208875
> + * kBps. With 32-bit symbols, we have 312500 kHz symbol clock. The value 1000000
> + * does not match the symbol clock, the port clock (not even if you think in
> + * terms of a byte clock), nor the data bandwidth. It only matches the link bit
> + * rate in units of 10000 bps.
> + *
> + * Note that protocol layers above the DPRX link level considered here can
> + * further limit the maximum data rate. Such layers are the MST topology (with
> + * limits on the link between the source and first branch device as well as on
> + * the whole MST path until the DPRX link) and (Thunderbolt) DP tunnels -
> + * which in turn can encapsulate an MST link with its own limit - with each
> + * SST or MST encapsulated tunnel sharing the BW of a tunnel group.
> + *
> + * TODO: Add support for querying the max data rate with the above limits as
> + * well.
> + *
> + * Returns the maximum data rate in kBps units.
> + */
> +int drm_dp_max_dprx_data_rate(int max_link_rate, int max_lanes)
> +{
> +	int ch_coding_efficiency =
> +		drm_dp_bw_channel_coding_efficiency(drm_dp_is_uhbr_rate(max_link_rate));
> +	int max_link_rate_kbps = max_link_rate * 10;

That x10 value seems rather pointless.

> +
> +	/*
> +	 * UHBR rates always use 128b/132b channel encoding, and have
> +	 * 97.71% data bandwidth efficiency. Consider max_link_rate the
> +	 * link bit rate in units of 10000 bps.
> +	 */
> +	/*
> +	 * Lower than UHBR rates always use 8b/10b channel encoding, and have
> +	 * 80% data bandwidth efficiency for SST non-FEC. However, this turns
> +	 * out to be a nop by coincidence:
> +	 *
> +	 *	int max_link_rate_kbps = max_link_rate * 10;
> +	 *	max_link_rate_kbps = DIV_ROUND_DOWN_ULL(max_link_rate_kbps * 8, 10);
> +	 *	max_link_rate = max_link_rate_kbps / 8;
> +	 */

Not sure why we are repeating the nuts and bolts detils in the
comments so much? Doesn't drm_dp_bw_channel_coding_efficiency()
explain all this already?

> +	return DIV_ROUND_DOWN_ULL(mul_u32_u32(max_link_rate_kbps * max_lanes,
> +					      ch_coding_efficiency),
> +				  1000000 * 8);
> +}
> +EXPORT_SYMBOL(drm_dp_max_dprx_data_rate);
> diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
> index 863b2e7add29e..454ae7517419a 100644
> --- a/include/drm/display/drm_dp_helper.h
> +++ b/include/drm/display/drm_dp_helper.h
> @@ -813,4 +813,6 @@ int drm_dp_bw_overhead(int lane_count, int hactive,
>  		       int bpp_x16, unsigned long flags);
>  int drm_dp_bw_channel_coding_efficiency(bool is_uhbr);
>  
> +int drm_dp_max_dprx_data_rate(int max_link_rate, int max_lanes);
> +
>  #endif /* _DRM_DP_HELPER_H_ */
> -- 
> 2.39.2

-- 
Ville Syrjälä
Intel