Reviewed-by: Lyude Paul <lyude@xxxxxxxxxx>
On Tue, 2023-11-07 at 02:14 +0200, Imre Deak wrote:
> Add helpers drivers can use to calculate the BW allocation overhead -
> due to SSC, FEC, DSC and data alignment on symbol cycles - and the
> channel coding efficiency - due to the 8b/10b, 128b/132b encoding. On
> 128b/132b links the FEC overhead is part of the coding efficiency, so
> not accounted for in the BW allocation overhead.
>
> The drivers can use these functions to calculate a ratio, controlling
> the stream symbol insertion rate of the source device in each SST TU
> or MST MTP frame. Drivers can calculate this
>
> m/n = (pixel_data_rate * drm_dp_bw_overhead()) /
> (link_data_rate * drm_dp_bw_channel_coding_efficiency())
>
> ratio for a given link and pixel stream and with that the
>
> slots_per_mtp = CEIL(64 * m / n)
>
> allocated slots per MTP for the stream in a link frame and with
> that the
>
> pbn = slots_per_mtp * drm_mst_get_pbn_divider()
>
> allocated PBNs for the stream on the MST link path.
>
> Take drm_dp_bw_overhead() into use in drm_dp_calc_pbn_mode(), for
> drivers calculating the PBN value directly.
>
> v2:
> - Add dockbook description to drm_dp_bw_channel_coding_efficiency().
> (LKP).
> - Clarify the way m/n ratio is calculated in the commit log.
> v3:
> - Fix compile breakage for !CONFIG_BACKLIGHT_CLASS_DEVICE. (LKP)
> - Account for FEC_PM overhead (+ 0.0015625 %), add comment
> with the formula to calculate the total FEC overhead. (Ville)
> v4:
> - Rename DRM_DP_OVERHEAD_SSC to DRM_DP_OVERHEAD_SSC_REF_CLK. (Ville)
> v5:
> - Clarify in the commit log what MTP means.
> - Simplify the commit log's formula to calculate PBN.
>
> Cc: Lyude Paul <lyude@xxxxxxxxxx>
> Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> Cc: kernel test robot <lkp@xxxxxxxxx>
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@xxxxxxxxx> (v2)
> Acked-by: Maxime Ripard <mripard@xxxxxxxxxx>
> Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx>
> ---
> drivers/gpu/drm/display/drm_dp_helper.c | 132 ++++++++++++++++++
> drivers/gpu/drm/display/drm_dp_mst_topology.c | 23 ++-
> include/drm/display/drm_dp_helper.h | 11 ++
> 3 files changed, 160 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> index e5d7970a9ddd0..72ba9ae89f862 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -3900,3 +3900,135 @@ int drm_panel_dp_aux_backlight(struct drm_panel *panel, struct drm_dp_aux *aux)
> EXPORT_SYMBOL(drm_panel_dp_aux_backlight);
>
> #endif
> +
> +/* See DP Standard v2.1 2.6.4.4.1.1, 2.8.4.4, 2.8.7 */
> +static int drm_dp_link_symbol_cycles(int lane_count, int pixels, int bpp_x16,
> + int symbol_size, bool is_mst)
> +{
> + int cycles = DIV_ROUND_UP(pixels * bpp_x16, 16 * symbol_size * lane_count);
> + int align = is_mst ? 4 / lane_count : 1;
> +
> + return ALIGN(cycles, align);
> +}
> +
> +static int drm_dp_link_dsc_symbol_cycles(int lane_count, int pixels, int slice_count,
> + int bpp_x16, int symbol_size, bool is_mst)
> +{
> + int slice_pixels = DIV_ROUND_UP(pixels, slice_count);
> + int slice_data_cycles = drm_dp_link_symbol_cycles(lane_count, slice_pixels,
> + bpp_x16, symbol_size, is_mst);
> + int slice_eoc_cycles = is_mst ? 4 / lane_count : 1;
> +
> + return slice_count * (slice_data_cycles + slice_eoc_cycles);
> +}
> +
> +/**
> + * drm_dp_bw_overhead - Calculate the BW overhead of a DP link stream
> + * @lane_count: DP link lane count
> + * @hactive: pixel count of the active period in one scanline of the stream
> + * @dsc_slice_count: DSC slice count if @flags/DRM_DP_LINK_BW_OVERHEAD_DSC is set
> + * @bpp_x16: bits per pixel in .4 binary fixed point
> + * @flags: DRM_DP_OVERHEAD_x flags
> + *
> + * Calculate the BW allocation overhead of a DP link stream, depending
> + * on the link's
> + * - @lane_count
> + * - SST/MST mode (@flags / %DRM_DP_OVERHEAD_MST)
> + * - symbol size (@flags / %DRM_DP_OVERHEAD_UHBR)
> + * - FEC mode (@flags / %DRM_DP_OVERHEAD_FEC)
> + * - SSC/REF_CLK mode (@flags / %DRM_DP_OVERHEAD_SSC_REF_CLK)
> + * as well as the stream's
> + * - @hactive timing
> + * - @bpp_x16 color depth
> + * - compression mode (@flags / %DRM_DP_OVERHEAD_DSC).
> + * Note that this overhead doesn't account for the 8b/10b, 128b/132b
> + * channel coding efficiency, for that see
> + * @drm_dp_link_bw_channel_coding_efficiency().
> + *
> + * Returns the overhead as 100% + overhead% in 1ppm units.
> + */
> +int drm_dp_bw_overhead(int lane_count, int hactive,
> + int dsc_slice_count,
> + int bpp_x16, unsigned long flags)
> +{
> + int symbol_size = flags & DRM_DP_BW_OVERHEAD_UHBR ? 32 : 8;
> + bool is_mst = flags & DRM_DP_BW_OVERHEAD_MST;
> + u32 overhead = 1000000;
> + int symbol_cycles;
> +
> + /*
> + * DP Standard v2.1 2.6.4.1
> + * SSC downspread and ref clock variation margin:
> + * 5300ppm + 300ppm ~ 0.6%
> + */
> + if (flags & DRM_DP_BW_OVERHEAD_SSC_REF_CLK)
> + overhead += 6000;
> +
> + /*
> + * DP Standard v2.1 2.6.4.1.1, 3.5.1.5.4:
> + * FEC symbol insertions for 8b/10b channel coding:
> + * After each 250 data symbols on 2-4 lanes:
> + * 250 LL + 5 FEC_PARITY_PH + 1 CD_ADJ (256 byte FEC block)
> + * After each 2 x 250 data symbols on 1 lane:
> + * 2 * 250 LL + 11 FEC_PARITY_PH + 1 CD_ADJ (512 byte FEC block)
> + * After 256 (2-4 lanes) or 128 (1 lane) FEC blocks:
> + * 256 * 256 bytes + 1 FEC_PM
> + * or
> + * 128 * 512 bytes + 1 FEC_PM
> + * (256 * 6 + 1) / (256 * 250) = 2.4015625 %
> + */
> + if (flags & DRM_DP_BW_OVERHEAD_FEC)
> + overhead += 24016;
> +
> + /*
> + * DP Standard v2.1 2.7.9, 5.9.7
> + * The FEC overhead for UHBR is accounted for in its 96.71% channel
> + * coding efficiency.
> + */
> + WARN_ON((flags & DRM_DP_BW_OVERHEAD_UHBR) &&
> + (flags & DRM_DP_BW_OVERHEAD_FEC));
> +
> + if (flags & DRM_DP_BW_OVERHEAD_DSC)
> + symbol_cycles = drm_dp_link_dsc_symbol_cycles(lane_count, hactive,
> + dsc_slice_count,
> + bpp_x16, symbol_size,
> + is_mst);
> + else
> + symbol_cycles = drm_dp_link_symbol_cycles(lane_count, hactive,
> + bpp_x16, symbol_size,
> + is_mst);
> +
> + return DIV_ROUND_UP_ULL(mul_u32_u32(symbol_cycles * symbol_size * lane_count,
> + overhead * 16),
> + hactive * bpp_x16);
> +}
> +EXPORT_SYMBOL(drm_dp_bw_overhead);
> +
> +/**
> + * drm_dp_bw_channel_coding_efficiency - Get a DP link's channel coding efficiency
> + * @is_uhbr: Whether the link has a 128b/132b channel coding
> + *
> + * Return the channel coding efficiency of the given DP link type, which is
> + * either 8b/10b or 128b/132b (aka UHBR). The corresponding overhead includes
> + * the 8b -> 10b, 128b -> 132b pixel data to link symbol conversion overhead
> + * and for 128b/132b any link or PHY level control symbol insertion overhead
> + * (LLCP, FEC, PHY sync, see DP Standard v2.1 3.5.2.18). For 8b/10b the
> + * corresponding FEC overhead is BW allocation specific, included in the value
> + * returned by drm_dp_bw_overhead().
> + *
> + * Returns the efficiency in the 100%/coding-overhead% ratio in
> + * 1ppm units.
> + */
> +int drm_dp_bw_channel_coding_efficiency(bool is_uhbr)
> +{
> + if (is_uhbr)
> + return 967100;
> + else
> + /*
> + * Note that on 8b/10b MST the efficiency is only
> + * 78.75% due to the 1 out of 64 MTPH packet overhead,
> + * not accounted for here.
> + */
> + return 800000;
> +}
> +EXPORT_SYMBOL(drm_dp_bw_channel_coding_efficiency);
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index cc0a8fe84d290..4d72c9a32026e 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -4726,17 +4726,28 @@ EXPORT_SYMBOL(drm_dp_check_act_status);
> int drm_dp_calc_pbn_mode(int clock, int 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 *= (1006/1000)
> - * peak_kbps *= (64/54)
> - * peak_kbps *= 8 convert to bytes
> + * peak_kbps = clock * bpp / 16
> + * peak_kbps *= SSC overhead / 1000000
> + * peak_kbps /= 8 convert to Kbytes
> + * peak_kBps *= (64/54) / 1000 convert to PBN
> */
> - return DIV_ROUND_UP_ULL(mul_u32_u32(clock * bpp, 64 * 1006 >> 4),
> - 1000 * 8 * 54 * 1000);
> + /*
> + * TODO: Use the actual link and mode parameters to calculate
> + * the overhead. For now it's assumed that these are
> + * 4 link lanes, 4096 hactive pixels, which don't add any
> + * significant data padding overhead and that there is no DSC
> + * or FEC overhead.
> + */
> + int overhead = drm_dp_bw_overhead(4, 4096, 0, bpp,
> + DRM_DP_BW_OVERHEAD_MST |
> + DRM_DP_BW_OVERHEAD_SSC_REF_CLK);
> +
> + return DIV64_U64_ROUND_UP(mul_u32_u32(clock * bpp, 64 * overhead >> 4),
> + 1000000ULL * 8 * 54 * 1000);
> }
> EXPORT_SYMBOL(drm_dp_calc_pbn_mode);
>
> diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
> index da94932f4262b..caee29d28463c 100644
> --- a/include/drm/display/drm_dp_helper.h
> +++ b/include/drm/display/drm_dp_helper.h
> @@ -788,4 +788,15 @@ bool drm_dp_downstream_rgb_to_ycbcr_conversion(const u8 dpcd[DP_RECEIVER_CAP_SIZ
> const u8 port_cap[4], u8 color_spc);
> int drm_dp_pcon_convert_rgb_to_ycbcr(struct drm_dp_aux *aux, u8 color_spc);
>
> +#define DRM_DP_BW_OVERHEAD_MST BIT(0)
> +#define DRM_DP_BW_OVERHEAD_UHBR BIT(1)
> +#define DRM_DP_BW_OVERHEAD_SSC_REF_CLK BIT(2)
> +#define DRM_DP_BW_OVERHEAD_FEC BIT(3)
> +#define DRM_DP_BW_OVERHEAD_DSC BIT(4)
> +
> +int drm_dp_bw_overhead(int lane_count, int hactive,
> + int dsc_slice_count,
> + int bpp_x16, unsigned long flags);
> +int drm_dp_bw_channel_coding_efficiency(bool is_uhbr);
> +
> #endif /* _DRM_DP_HELPER_H_ */
--
Cheers,
Lyude Paul (she/her)
Software Engineer at Red Hat
