Re: [v4 2/2] drm/i915/audio: Prune ELD SADs based on HDMI audio bandwidth

["Nautiyal, Ankit K" <ankit.k.nautiyal@intel.com>]

On 8/13/2026 5:30 PM, Chaitanya Kumar Borah wrote:
> Add bandwidth check to determine whether a given audio sample rate and
> channel count can be carried within the hblank period for HDMI TMDS mode.
>
> Use this check to prune unsupported sample rates from each SAD in the
> ELD during intel_audio_compute_config(). SADs with no remaining
> supported rates are removed entirely.
>
> Sample rates are pruned rather than channel counts, since compressed
> formats (e.g. AC-3) are associated with specific channel counts.
>
> v2:
>   - Use DIV64_U64_ROUND_UP() instead of DIV_ROUND_UP_ULL() to avoid
>     do_div() truncating the 64-bit divisor to 32-bit, which caused
>     audio_packets_line to be wildly inflated and all SADs to be pruned.
>   - Guard intel_audio_hdmi_eld_compute_config() against HDMI FRL modes.
>     (Remove it when BW calculations for FRL are added.)
>
> v3:
>   - Rebase
>
> v4:
>   - Rework SAD removal into a single-pass, two-pointer compaction
>     instead of memmove-ing the tail on every removal. (Jani)
>
> BSpec: 68944
> Cc: Kai Vehmanen <kai.vehmanen@linux.intel.com>
> Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Vinod Govindapillai <vinod.govindapillai@intel.com>
> Cc: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Assisted-by: GitHub-Copilot:claude-opus-4.6
> Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
> ---
>   drivers/gpu/drm/i915/display/intel_audio.c | 156 +++++++++++++++++++++
>   1 file changed, 156 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_audio.c b/drivers/gpu/drm/i915/display/intel_audio.c
> index eae76e961105..9eb6867fa3f3 100644
> --- a/drivers/gpu/drm/i915/display/intel_audio.c
> +++ b/drivers/gpu/drm/i915/display/intel_audio.c
> @@ -39,6 +39,7 @@
>   #include "intel_display_types.h"
>   #include "intel_display_wa.h"
>   #include "intel_dp.h"
> +#include "intel_hdmi.h"
>   #include "intel_lpe_audio.h"
>   
>   /**
> @@ -697,6 +698,158 @@ static void ibx_audio_codec_enable(struct intel_encoder *encoder,
>   	mutex_unlock(&display->audio.mutex);
>   }
>   
> +static bool hdmi_audio_rate_supported(const struct intel_crtc_state *crtc_state,
> +				      int available_tmds,
> +				      int audio_rate, int channels)
> +{
> +	const struct drm_display_mode *mode = &crtc_state->hw.adjusted_mode;
> +	int pixel_clk_max_hz;
> +	int audio_pkt_factor;
> +	u64 audio_pkt_rate_x4_x1000;
> +	int audio_packets_line;
> +	int hblank_overhead;
> +	int required_tmds;
This can be names as hblank_audio_min the HDMI spec, and bspec both 
mention this to denote required tribytes
> +
> +	/*
> +	 * Part 2: Calculate TMDS clock cycles required for Audio Bandwidth
> +	 *
> +	 * Step 1: pixelclk_max = nominal_pixel_rate * (1 + 0.5%)
> +	 * crtc_clock (kHz) * 1000 * 1.005 = crtc_clock * 1005 (Hz)
> +	 */
> +	pixel_clk_max_hz = mode->crtc_clock * 1005;
> +
> +	/*
> +	 * Steps 3-4: Audio Packet Rate.
> +	 *   R_AP = (audio_rate * AP + 2 * acrrate_max) * (1 + 1000 / 1e6)
> +	 *        = (audio_rate * AP + 2*1500) * 1.001
> +	 *
> +	 * AP = 0.25 (2ch) or 1.0 (3-8ch); acrrate_max = 1500 Hz (max ACR
> +	 * packet transmission rate per HDMI spec)
> +	 *
> +	 * Scale by 4*1000 to stay integer:
> +	 *  x4: eliminates AP=0.25 -> audio_pkt_factor=1(2ch) or 4(3-8ch),
> +	 *      scaled acrrate_max: 2 * 1500 * 4 = 12000
> +	 *  x1000: eliminates 1.001 -> *1000*1.001 = *1001
> +	 *
> +	 * R_AP * 4 * 1000 = (audio_rate * audio_pkt_factor + 12000) * 1001
> +	 */
> +	audio_pkt_factor = (channels <= 2) ? 1 : 4;
> +	audio_pkt_rate_x4_x1000 = (u64)(audio_rate * audio_pkt_factor + 12000) * 1001;
> +
> +	/*
> +	 * Steps 2+5-6: Audio packets per line.
> +	 *   AudioPackets_Line = CEIL[R_AP * htotal / f_pixelclk_max]
> +	 *
> +	 * With audio_pkt_rate_x4_x1000 = R_AP * 4 * 1000:
> +	 *   = CEIL[audio_pkt_rate_x4_x1000 * htotal / (4 * 1000 * pixel_clk_max_hz)]
> +	 */
> +	audio_packets_line = DIV64_U64_ROUND_UP(audio_pkt_rate_x4_x1000 * mode->htotal,
> +						(u64)4 * 1000 * pixel_clk_max_hz);
> +
> +	/*
> +	 * Steps 7-9: Hblank overhead.
> +	 * Standard:  2*dip_guardband + 2*control_period + video_guardband
> +	 *          = 2*2 + 2*12 + 2 = 30
> +	 * HDCP 1.x:  rekey_period + dip_guardband + control_period + video_guardband
> +	 *          = 58 + 2 + 12 + 2 = 74
> +	 *
> +	 * Always use HDCP 1.x worst case (74) since HDCP can be toggled
> +	 * via fastset without compute_config.
> +	 */
> +	hblank_overhead = 74;
> +
> +	/*
> +	 * Step 10: Required TMDS cycles for Audio.
> +	 *  32 TMDS clock cycles per audio packet.
> +	 *   Hblank_audio_min = 32 * AudioPackets_Line + Hblank_overhead
> +	 */
> +	required_tmds = 32 * audio_packets_line + hblank_overhead;
> +
> +	/*
> +	 * Part 3: audio supported if Hblank_audio_min <= TB_blank and
> +	 * audio packets per line <= Maximum allowed packets per line (18)
This can be a macro HDMI_TMDS_AUDIO_PACKETS_LINE_MAX

> +	 */
> +
> +

The extra blank line can be removed.


> 	return required_tmds <= available_tmds && audio_packets_line <= 18;
> +}
> +
> +static void intel_audio_hdmi_eld_compute_config(struct intel_crtc_state *crtc_state)
> +{
> +	static const int sad_freqs[] = {
> +		32000, 44100, 48000, 88200, 96000, 176400, 192000
> +	};
> +	const struct drm_display_mode *mode = &crtc_state->hw.adjusted_mode;
> +	int hblank = mode->htotal - mode->hdisplay;
> +	int bpc = crtc_state->pipe_bpp / 3;
> +	int ycbcr_420_divider = (crtc_state->output_format == INTEL_OUTPUT_FORMAT_YCBCR420) ? 2 : 1;
> +	int available_tmds;
> +	u8 *eld = crtc_state->eld;
> +	int mnl = drm_eld_mnl(eld);
> +	int sad_count = drm_eld_sad_count(eld);
> +	int src, dst = 0;
> +
> +	/* Only applies to HDMI TMDS, not FRL */
> +	if (intel_hdmi_is_frl(crtc_state->port_clock))
> +		return;

This can be dropped as of now we do not have FRL support. As we get FRL 
support, we shall use appropriate check here.


> +	/*
> +	 * Part 1: Calculate available TMDS clock cycles (TB_blank).
> +	 *
> +	 * TB_blank = CEILING[hblank * K_CD / K_420]
> +	 *
> +	 * K_CD = 1 for YCbCr4:2:2, bpc / 8 otherwise.
> +	 * K_420 = 2 for YCbCr4:2:0, 1 otherwise.
> +	 * Rearranged: CEILING[hblank * bpc / (8 * K_420)]
> +	 *
> +	 * TODO: As and when support for YCbCr4:2:2 is added, set bpc = 8
> +	 * to achieve K_CD = 1
> +	 */
> +	available_tmds = DIV_ROUND_UP(hblank * bpc, 8 * ycbcr_420_divider);
> +
> +	/*
> +	 * Walk all SADs once, keeping the ones with at least one supported
> +	 * rate and compacting them down to a contiguous [0, dst) range.
> +	 */
> +	for (src = 0; src < sad_count; src++) {
> +		int src_offset = DRM_ELD_CEA_SAD(mnl, src);
> +		int channels = (eld[src_offset] & 0x7) + 1;
> +		u8 freq_mask = eld[src_offset + 1];

Hmm can we use : drm_eld_sad_get() this takes struct cea_sad *cta_sad as 
input parameter and fills it.

we can then use cta_sad.channels and cta_sad.freq for max channels and 
freq_mask.

Overall the direction of the change looks good to me.


Regards,

Ankit

> +		u8 new_freq_mask = 0;
> +		int bit;
> +
> +		for (bit = 0; bit < 7; bit++) {
> +			if (!(freq_mask & BIT(bit)))
> +				continue;
> +			if (hdmi_audio_rate_supported(crtc_state, available_tmds,
> +						      sad_freqs[bit], channels))
> +				new_freq_mask |= BIT(bit);
> +		}
> +
> +		if (!new_freq_mask)
> +			continue;
> +
> +		eld[src_offset + 1] = new_freq_mask;
> +
> +		if (dst != src)
> +			memmove(&eld[DRM_ELD_CEA_SAD(mnl, dst)],
> +				&eld[src_offset], 3);
> +		dst++;
> +	}
> +
> +	/* Clear the now-unused tail */
> +	if (dst != sad_count)
> +		memset(&eld[DRM_ELD_CEA_SAD(mnl, dst)], 0, (sad_count - dst) * 3);
> +
> +	sad_count = dst;
> +
> +	/* Update SAD count in ELD header */
> +	eld[DRM_ELD_SAD_COUNT_CONN_TYPE] &= ~DRM_ELD_SAD_COUNT_MASK;
> +	eld[DRM_ELD_SAD_COUNT_CONN_TYPE] |= sad_count << DRM_ELD_SAD_COUNT_SHIFT;
> +
> +	/* Recalculate baseline ELD length (in dwords) */
> +	eld[DRM_ELD_BASELINE_ELD_LEN] =
> +		DIV_ROUND_UP(drm_eld_calc_baseline_block_size(eld), 4);
> +}
> +
>   static
>   bool intel_audio_needs_cpu_transcoder_id(const struct intel_crtc_state *crtc_state)
>   {
> @@ -725,6 +878,9 @@ bool intel_audio_compute_config(struct intel_encoder *encoder,
>   	BUILD_BUG_ON(sizeof(crtc_state->eld) != sizeof(connector->eld));
>   	memcpy(crtc_state->eld, connector->eld, sizeof(crtc_state->eld));
>   
> +	if (intel_crtc_has_type(crtc_state, INTEL_OUTPUT_HDMI))
> +		intel_audio_hdmi_eld_compute_config(crtc_state);
> +
>   	crtc_state->eld[6] = drm_av_sync_delay(connector, adjusted_mode) / 2;
>   	mutex_unlock(&connector->eld_mutex);
>