[PATCH v4 09/30] drm/dp: Add helpers to calculate the link BW overhead
Ville Syrjälä
ville.syrjala at linux.intel.com
Tue Nov 7 16:24:04 UTC 2023
On Tue, Nov 07, 2023 at 12:28:24AM +0200, Imre Deak wrote:
> On Mon, Nov 06, 2023 at 11:31:51PM +0200, Ville Syrjälä wrote:
> > On Mon, Oct 30, 2023 at 05:58:22PM +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
> > >
> > > mtp_count = CEIL(64 * m / n)
> > >
> > > allocated MTPs for the stream in a link frame and
> > >
> > > pbn = CEIL(64 * dm_mst_get_pbn_divider() * m / n)
> > >
> > > 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)
> > >
> > > Cc: Lyude Paul <lyude at redhat.com>
> > > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > Cc: kernel test robot <lkp at intel.com>
> > > Cc: dri-devel at lists.freedesktop.org
> > > Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com> (v2)
> > > Signed-off-by: Imre Deak <imre.deak at intel.com>
> > > ---
> > > 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;
> >
> > Hmm. Shouldn't we be multiplying the overheads, not add them up?
> > Though I suppose in this case the error won't be huge.
>
> Yes logically these should be multiplied. I added them here, because
> that's what the spec does with explicitly mentioning the 3% total
> overhead.
>
> I presume it's a simplification, maybe because the 0.6%
> SSC+REF_CLK overhead is already a rounded-up value, the exact one being
> only 0.53% for SSC and 0.03% for REF_CLK.
Yeah, they also didn't bother multiplying those two together
and just added them together. But for the rest they rather just
give rough estimates instead of exact numbers.
However
((256*256+1)/(256*250))*1.0053 = ~1.02944
vs.
1.0+0.006+0.024016 = 1.030016
so yeah, it does look like we should be safe with just
adding them up (and the rough 3% total the spec mentions
would also be safe).
>
> > > +
> > > + /*
> > > + * 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_ */
> > > --
> > > 2.39.2
> >
> > --
> > Ville Syrjälä
> > Intel
--
Ville Syrjälä
Intel
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