Re: [PATCHv3] drm/i915/dp: Guarantee a minimum HBlank time

[Jani Nikula <jani.nikula@intel.com>]

On Mon, 28 Oct 2024, Arun R Murthy <arun.r.murthy@intel.com> wrote:
> Mandate a minimum Hblank symbol cycle count between BS and BE in 8b/10b
> MST and 12b/132b mode.
> Spec: DP2.1a
>
> v2: Affine calculation/updation of min HBlank to dp_mst (Jani)
> v3: moved min_hblank from struct intel_dp to intel_crtc_state (Jani)
>
> Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_atomic.c   |  1 +
>  drivers/gpu/drm/i915/display/intel_crtc.c     |  1 +
>  .../drm/i915/display/intel_display_types.h    |  1 +
>  drivers/gpu/drm/i915/display/intel_dp_mst.c   | 28 +++++++++++++++++++
>  drivers/gpu/drm/i915/i915_reg.h               |  4 +++
>  5 files changed, 35 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_atomic.c b/drivers/gpu/drm/i915/display/intel_atomic.c
> index 03dc54c802d3..24aa079efcad 100644
> --- a/drivers/gpu/drm/i915/display/intel_atomic.c
> +++ b/drivers/gpu/drm/i915/display/intel_atomic.c
> @@ -278,6 +278,7 @@ intel_crtc_duplicate_state(struct drm_crtc *crtc)
>  	crtc_state->dsb_color_vblank = NULL;
>  	crtc_state->dsb_commit = NULL;
>  	crtc_state->use_dsb = false;
> +	crtc_state->min_hblank = 0;
>  
>  	return &crtc_state->uapi;
>  }
> diff --git a/drivers/gpu/drm/i915/display/intel_crtc.c b/drivers/gpu/drm/i915/display/intel_crtc.c
> index 3c9168a57f38..472f3010bce4 100644
> --- a/drivers/gpu/drm/i915/display/intel_crtc.c
> +++ b/drivers/gpu/drm/i915/display/intel_crtc.c
> @@ -184,6 +184,7 @@ void intel_crtc_state_reset(struct intel_crtc_state *crtc_state,
>  	crtc_state->scaler_state.scaler_id = -1;
>  	crtc_state->mst_master_transcoder = INVALID_TRANSCODER;
>  	crtc_state->max_link_bpp_x16 = INT_MAX;
> +	crtc_state->min_hblank = 0;
>  }
>  
>  static struct intel_crtc *intel_crtc_alloc(void)
> diff --git a/drivers/gpu/drm/i915/display/intel_display_types.h b/drivers/gpu/drm/i915/display/intel_display_types.h
> index 2bb1fa64da2f..7f631fef128e 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_types.h
> +++ b/drivers/gpu/drm/i915/display/intel_display_types.h
> @@ -1075,6 +1075,7 @@ struct intel_crtc_state {
>  
>  	int max_link_bpp_x16;	/* in 1/16 bpp units */
>  	int pipe_bpp;		/* in 1 bpp units */
> +	int min_hblank;		/* min HBlank for DP2.1 */
>  	struct intel_link_m_n dp_m_n;
>  
>  	/* m2_n2 for eDP downclock */
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> index 1a2ff3e1cb68..2e088760a23a 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> @@ -161,6 +161,28 @@ static int intel_dp_mst_dsc_get_slice_count(const struct intel_connector *connec
>  					    num_joined_pipes);
>  }
>  
> +static void intel_dp_mst_compute_min_hblank(struct intel_crtc_state *crtc_state,
> +					    struct intel_connector *intel_connector,

Always name struct intel_connector *connector, not
intel_connector. That's what the style has been for years now, though
not everything's been converted.

> +					    int bpp_x16)
> +{
> +	struct intel_encoder *intel_encoder = intel_connector->encoder;

Ditto, just encoder.

> +	struct intel_display *intel_display = to_intel_display(intel_encoder);

Just display. Always. Everywhere. Make this the first local variable
whenever possible.

> +	const struct drm_display_mode *adjusted_mode =
> +					&crtc_state->hw.adjusted_mode;
> +	u32 symbol_size = intel_dp_is_uhbr(crtc_state) ? 32 : 8;
> +	u32 hblank;

Looks like these should be ints.

> +
> +	if (DISPLAY_VER(intel_display) < 20)
> +		return;
> +
> +	/* Calculate min Hblank Link Layer Symbol Cycle Count for 8b/10b MST & 128b/132b */
> +	hblank = DIV_ROUND_UP((DIV_ROUND_UP(adjusted_mode->htotal - adjusted_mode->hdisplay, 4) * bpp_x16), symbol_size);

Okay so I didn't really think this through, but 1) do we lose precision
by dividing by 4 first, and multiplying by bpp_x16 next, and 2) how does
this take into account that bpp_x16 is in fact the x16 value?

> +	if (intel_dp_is_uhbr(crtc_state))
> +		crtc_state->min_hblank = (hblank > 5) ? hblank : 5;
> +	else
> +		crtc_state->min_hblank = (hblank > 3) ? hblank : 3;

These look like hand-rolled max() that needlessly duplicate both hblank
and the constant.

> +}
> +
>  static int intel_dp_mst_find_vcpi_slots_for_bpp(struct intel_encoder *encoder,
>  						struct intel_crtc_state *crtc_state,
>  						int max_bpp,
> @@ -238,6 +260,8 @@ static int intel_dp_mst_find_vcpi_slots_for_bpp(struct intel_encoder *encoder,
>  		remote_bw_overhead = intel_dp_mst_bw_overhead(crtc_state, connector,
>  							      true, dsc_slice_count, link_bpp_x16);
>  
> +		intel_dp_mst_compute_min_hblank(crtc_state, connector, link_bpp_x16);
> +
>  		intel_dp_mst_compute_m_n(crtc_state, connector,
>  					 local_bw_overhead,
>  					 link_bpp_x16,
> @@ -1292,6 +1316,10 @@ static void intel_mst_enable_dp(struct intel_atomic_state *state,
>  			       TRANS_DP2_VFREQ_PIXEL_CLOCK(crtc_clock_hz & 0xffffff));
>  	}
>  
> +	if (DISPLAY_VER(dev_priv) >= 20)

Always use the display local var when possible.

DISPLAY_VER(display)

> +		intel_de_write(dev_priv, DP_MIN_HBLANK_CTL(dev_priv, trans),
> +			       pipe_config->min_hblank);

intel_de_write(display, .... DP_MIN_HBLANK_CTL(display, trans) ...

> +
>  	enable_bs_jitter_was(pipe_config);
>  
>  	intel_ddi_enable_transcoder_func(encoder, pipe_config);
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 405f409e9761..8aeed48f9170 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -1147,6 +1147,10 @@
>  #define _TRANS_MULT_B		0x6102c
>  #define TRANS_MULT(dev_priv, trans)	_MMIO_TRANS2(dev_priv, (trans), _TRANS_MULT_A)
>  
> +#define _DP_MIN_HBLANK_CTL_A	0x600ac
> +#define _DP_MIN_HBLANK_CTL_B	0x610ac
> +#define DP_MIN_HBLANK_CTL(dev_priv, trans)	_MMIO_TRANS2(dev_priv, (trans), _DP_MIN_HBLANK_CTL_A)
> +
>  /* VGA port control */
>  #define ADPA			_MMIO(0x61100)
>  #define PCH_ADPA                _MMIO(0xe1100)

-- 
Jani Nikula, Intel