[PATCH v5 09/30] drm/dp: Add helpers to calculate the link BW overhead
Imre Deak
imre.deak at intel.com
Tue Nov 7 00:14:58 UTC 2023
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 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)
Acked-by: Maxime Ripard <mripard at kernel.org>
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;
+
+ /*
+ * 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
More information about the dri-devel
mailing list