Re: [Intel-gfx] [PATCH v4 09/25] drm/i915: Add helpers for BW management on shared display links

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Imre Deak <imre.deak@intel.com>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH v4 09/25] drm/i915: Add helpers for BW management on shared display links
Date: Fri, 15 Sep 2023 22:11:38 +0300	[thread overview]
Message-ID: <ZQSsal4rSD4XZ0cY@intel.com> (raw)
In-Reply-To: <20230915003354.857552-1-imre.deak@intel.com>

On Fri, Sep 15, 2023 at 03:33:54AM +0300, Imre Deak wrote:
> At the moment a modeset fails if the config computation of a pipe can't
> fit its required BW to the available link BW even though the limitation
> may be resolved by reducing the BW requirement of other pipes.
> 
> To improve the above this patch adds helper functions checking the
> overall BW limits after all CRTC states have been computed. If the check
> fails the maximum link bpp for a selected pipe will be reduced and all
> the CRTC states will be recomputed until either the overall BW limit
> check passes, or further bpp reduction is not possible (because all
> pipes/encoders sharing the link BW reached their minimum link bpp).
> 
> This change prepares for upcoming patches enabling the above BW
> management on FDI and MST links.
> 
> v2:
> - Rename intel_crtc_state::max_link_bpp to max_link_bpp_x16 and
>   intel_link_bw_limits::max_bpp to max_bpp_x16. (Jani)
> v3:
> - Add the helper functions in a separate patch. (Ville)
> - Add the functions to intel_link_bw.c instead of intel_atomic.c (Ville)
> - Return -ENOSPC instead of -EINVAL to userspace in case of a link BW
>   limit failure.
> v4:
> - Make intel_atomic_check_config() static.
> 
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Signed-off-by: Imre Deak <imre.deak@intel.com>
> ---
>  drivers/gpu/drm/i915/Makefile                 |   1 +
>  drivers/gpu/drm/i915/display/intel_crtc.c     |   1 +
>  drivers/gpu/drm/i915/display/intel_display.c  |  61 ++++-
>  .../drm/i915/display/intel_display_types.h    |   3 +-
>  drivers/gpu/drm/i915/display/intel_link_bw.c  | 226 ++++++++++++++++++
>  drivers/gpu/drm/i915/display/intel_link_bw.h  |  38 +++
>  6 files changed, 325 insertions(+), 5 deletions(-)
>  create mode 100644 drivers/gpu/drm/i915/display/intel_link_bw.c
>  create mode 100644 drivers/gpu/drm/i915/display/intel_link_bw.h
> 
> diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
> index 1b2e02e9d92cb..de4967c141f00 100644
> --- a/drivers/gpu/drm/i915/Makefile
> +++ b/drivers/gpu/drm/i915/Makefile
> @@ -268,6 +268,7 @@ i915-y += \
>  	display/intel_hotplug.o \
>  	display/intel_hotplug_irq.o \
>  	display/intel_hti.o \
> +	display/intel_link_bw.o \
>  	display/intel_load_detect.o \
>  	display/intel_lpe_audio.o \
>  	display/intel_modeset_lock.o \
> diff --git a/drivers/gpu/drm/i915/display/intel_crtc.c b/drivers/gpu/drm/i915/display/intel_crtc.c
> index 182c6dd64f47c..1eda6a9f19aa8 100644
> --- a/drivers/gpu/drm/i915/display/intel_crtc.c
> +++ b/drivers/gpu/drm/i915/display/intel_crtc.c
> @@ -175,6 +175,7 @@ void intel_crtc_state_reset(struct intel_crtc_state *crtc_state,
>  	crtc_state->hsw_workaround_pipe = INVALID_PIPE;
>  	crtc_state->scaler_state.scaler_id = -1;
>  	crtc_state->mst_master_transcoder = INVALID_TRANSCODER;
> +	crtc_state->max_link_bpp_x16 = INT_MAX;

Are we sure we won't end up doing random arithmetic with that which could
overflow?

>  }
>  
>  static struct intel_crtc *intel_crtc_alloc(void)
> diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> index fe3b6844e063d..0f30723a68cc0 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -87,6 +87,7 @@
>  #include "intel_frontbuffer.h"
>  #include "intel_hdmi.h"
>  #include "intel_hotplug.h"
> +#include "intel_link_bw.h"
>  #include "intel_lvds.h"
>  #include "intel_lvds_regs.h"
>  #include "intel_modeset_setup.h"
> @@ -4596,7 +4597,8 @@ intel_crtc_prepare_cleared_state(struct intel_atomic_state *state,
>  
>  static int
>  intel_modeset_pipe_config(struct intel_atomic_state *state,
> -			  struct intel_crtc *crtc)
> +			  struct intel_crtc *crtc,
> +			  const struct intel_link_bw_limits *limits)
>  {
>  	struct drm_i915_private *i915 = to_i915(crtc->base.dev);
>  	struct intel_crtc_state *crtc_state =
> @@ -4628,6 +4630,15 @@ intel_modeset_pipe_config(struct intel_atomic_state *state,
>  	if (ret)
>  		return ret;
>  
> +	crtc_state->max_link_bpp_x16 = limits->max_bpp_x16[crtc->pipe];
> +
> +	if (crtc_state->pipe_bpp > to_bpp_int(crtc_state->max_link_bpp_x16)) {
> +		drm_dbg_kms(&i915->drm,
> +			    "[CRTC:%d:%s] Link bpp limited to " BPP_X16_FMT "\n",
> +			    crtc->base.base.id, crtc->base.name,
> +			    BPP_X16_ARGS(crtc_state->max_link_bpp_x16));
> +	}
> +
>  	base_bpp = crtc_state->pipe_bpp;
>  
>  	/*
> @@ -6195,7 +6206,9 @@ static int intel_bigjoiner_add_affected_crtcs(struct intel_atomic_state *state)
>  	return 0;
>  }
>  
> -static int intel_atomic_check_config(struct intel_atomic_state *state)
> +static int intel_atomic_check_config(struct intel_atomic_state *state,
> +				     struct intel_link_bw_limits *limits,
> +				     enum pipe *failed_pipe)
>  {
>  	struct drm_i915_private *i915 = to_i915(state->base.dev);
>  	struct intel_crtc_state *new_crtc_state;
> @@ -6203,6 +6216,8 @@ static int intel_atomic_check_config(struct intel_atomic_state *state)
>  	int ret;
>  	int i;
>  
> +	*failed_pipe = INVALID_PIPE;
> +
>  	ret = intel_bigjoiner_add_affected_crtcs(state);
>  	if (ret)
>  		return ret;
> @@ -6228,7 +6243,7 @@ static int intel_atomic_check_config(struct intel_atomic_state *state)
>  		if (!new_crtc_state->hw.enable)
>  			continue;
>  
> -		ret = intel_modeset_pipe_config(state, crtc);
> +		ret = intel_modeset_pipe_config(state, crtc, limits);
>  		if (ret)
>  			break;
>  
> @@ -6237,9 +6252,47 @@ static int intel_atomic_check_config(struct intel_atomic_state *state)
>  			break;
>  	}
>  
> +	if (ret)
> +		*failed_pipe = crtc->pipe;
> +
>  	return ret;
>  }
>  
> +static int intel_atomic_check_config_and_link(struct intel_atomic_state *state)
> +{
> +	struct drm_i915_private *i915 = to_i915(state->base.dev);
> +	struct intel_link_bw_limits new_limits;
> +	struct intel_link_bw_limits old_limits;
> +	int ret;
> +
> +	intel_link_bw_init_limits(i915, &new_limits);
> +	old_limits = new_limits;
> +
> +	while (true) {
> +		enum pipe failed_pipe;
> +
> +		ret = intel_atomic_check_config(state, &new_limits,
> +						&failed_pipe);
> +		if (ret) {
> +			if (ret == -EINVAL &&
> +			    intel_link_bw_reset_pipe_limit_to_min(state,
> +								  &old_limits,
> +								  &new_limits,
> +								  failed_pipe))
> +				continue;
> +
> +			break;
> +		}
> +
> +		old_limits = new_limits;
> +
> +		ret = intel_link_bw_atomic_check(state, &new_limits);
> +		if (ret != -EAGAIN)
> +			break;
> +	}
> +
> +	return ret;
> +}
>  /**
>   * intel_atomic_check - validate state object
>   * @dev: drm device
> @@ -6284,7 +6337,7 @@ int intel_atomic_check(struct drm_device *dev,
>  			return ret;
>  	}
>  
> -	ret = intel_atomic_check_config(state);
> +	ret = intel_atomic_check_config_and_link(state);
>  	if (ret)
>  		goto fail;
>  
> diff --git a/drivers/gpu/drm/i915/display/intel_display_types.h b/drivers/gpu/drm/i915/display/intel_display_types.h
> index 966163ccbd7a3..514977ebc17b0 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_types.h
> +++ b/drivers/gpu/drm/i915/display/intel_display_types.h
> @@ -1189,7 +1189,8 @@ struct intel_crtc_state {
>  		u32 ctrl, div;
>  	} dsi_pll;
>  
> -	int pipe_bpp;
> +	int max_link_bpp_x16;	/* in 1/16 bpp units */
> +	int pipe_bpp;		/* in 1 bpp units */
>  	struct intel_link_m_n dp_m_n;
>  
>  	/* m2_n2 for eDP downclock */
> diff --git a/drivers/gpu/drm/i915/display/intel_link_bw.c b/drivers/gpu/drm/i915/display/intel_link_bw.c
> new file mode 100644
> index 0000000000000..22494772b9d59
> --- /dev/null
> +++ b/drivers/gpu/drm/i915/display/intel_link_bw.c
> @@ -0,0 +1,226 @@
> +// SPDX-License-Identifier: MIT
> +/*
> + * Copyright © 2023 Intel Corporation
> + */
> +
> +#include "i915_drv.h"
> +
> +#include "intel_atomic.h"
> +#include "intel_display_types.h"
> +#include "intel_dp_mst.h"
> +#include "intel_fdi.h"
> +#include "intel_link_bw.h"
> +
> +/**
> + * intel_link_bw_init_limits - initialize BW limits
> + * @i915: device instance
> + * @limits: link BW limits
> + *
> + * Initialize @limits.
> + */
> +void intel_link_bw_init_limits(struct drm_i915_private *i915, struct intel_link_bw_limits *limits)
> +{
> +	enum pipe pipe;
> +
> +	limits->min_bpp_pipes = 0;
> +	for_each_pipe(i915, pipe)
> +		limits->max_bpp_x16[pipe] = INT_MAX;
> +}
> +
> +/**
> + * intel_link_bw_compute_pipe_bpp - compute pipe bpp limited by max link bpp
> + * @crtc_state: the crtc state
> + *
> + * Compute the pipe bpp limited by the CRTC's maximum link bpp. Encoders can
> + * call this function during state computation in the simple case where the
> + * link bpp will always match the pipe bpp. This is the case for all non-DP
> + * encoders, while DP encoders will use a link bpp lower than pipe bpp in case
> + * of DSC compression.
> + *
> + * Returns %true in case of success, %false if pipe bpp would need to be
> + * reduced below its valid range.
> + */
> +bool intel_link_bw_compute_pipe_bpp(struct intel_crtc_state *crtc_state)
> +{
> +	int pipe_bpp = min(crtc_state->pipe_bpp,
> +			   to_bpp_int(crtc_state->max_link_bpp_x16));
> +
> +	pipe_bpp = rounddown(pipe_bpp, 2 * 3);
> +
> +	if (pipe_bpp < 6 * 3)
> +		return false;
> +
> +	crtc_state->pipe_bpp = pipe_bpp;
> +
> +	return true;
> +}
> +
> +/**
> + * intel_link_bw_reduce_bpp - reduce maximum link bpp for a selected pipe
> + * @state: atomic state
> + * @limits: link BW limits
> + * @pipe_mask: mask of pipes to select from
> + * @reason: explanation of why bpp reduction is needed
> + *
> + * Select the pipe from @pipe_mask with the biggest link bpp value and set the
> + * maximum of link bpp in @limits below this value. Modeset the selected pipe,
> + * so that its state will get recomputed.
> + *
> + * This function can be called to resolve a link's BW overallocation by reducing
> + * the link bpp of one pipe on the link and hence reducing the total link BW.
> + *
> + * Returns
> + *   - 0 in case of success
> + *   - %-ENOSPC if no pipe can further reduce its link bpp
> + *   - Other negative error, if modesetting the selected pipe failed
> + */
> +int intel_link_bw_reduce_bpp(struct intel_atomic_state *state,
> +			     struct intel_link_bw_limits *limits,
> +			     u8 pipe_mask,
> +			     const char *reason)
> +{
> +	struct drm_i915_private *i915 = to_i915(state->base.dev);
> +	enum pipe max_bpp_pipe = INVALID_PIPE;
> +	struct intel_crtc *crtc;
> +	int max_bpp = 0;
> +
> +	for_each_intel_crtc_in_pipe_mask(&i915->drm, crtc, pipe_mask) {
> +		struct intel_crtc_state *crtc_state;
> +		int pipe_bpp;
> +
> +		if (limits->min_bpp_pipes & BIT(crtc->pipe))
> +			continue;
> +
> +		crtc_state = intel_atomic_get_crtc_state(&state->base,
> +							 crtc);
> +		if (IS_ERR(crtc_state))
> +			return PTR_ERR(crtc_state);
> +
> +		if (crtc_state->dsc.compression_enable)
> +			pipe_bpp = crtc_state->dsc.compressed_bpp;
> +		else
> +			pipe_bpp = crtc_state->pipe_bpp;

That sounds like it should be 'link_bpp'.

Hmm. How does this work with 4:2:0 output?

> +
> +		if (pipe_bpp > max_bpp) {
> +			max_bpp = pipe_bpp;
> +			max_bpp_pipe = crtc->pipe;
> +		}
> +	}
> +
> +	if (max_bpp_pipe == INVALID_PIPE)
> +		return -ENOSPC;
> +
> +	limits->max_bpp_x16[max_bpp_pipe] = to_bpp_x16(max_bpp) - 1;
> +
> +	return intel_modeset_pipes_in_mask_early(state, reason,
> +						 BIT(max_bpp_pipe));

So we always reduce just for one pipe at a time. I suppose that does
lead to the most optimal bandwidth usage, albeit with perhaps a
slighly slower convergence.

> +}
> +
> +/**
> + * intel_link_bw_reset_pipe_limit_to_min - reset BW limit for a pipe to its minimum
> + * @state: atomic state
> + * @old_limits: link BW limits
> + * @new_limits: link BW limits
> + * @pipe: pipe
> + *
> + * Reset the link bpp limit for @pipe in @new_limits to its value in
> + * @old_limits and mark this limit as the minimum. This function must be
> + * called after a pipe's compute config function failed, @old_limits
> + * containing the bpp limit with which compute config previously passed.
> + *
> + * The function will fail if setting a minimum is not possible, either
> + * because the old and new limits match (and so would lead to a pipe compute
> + * config failure) or the limit is already at the minimum.
> + *
> + * Returns %true in case of success.
> + */
> +bool
> +intel_link_bw_reset_pipe_limit_to_min(struct intel_atomic_state *state,
> +				       const struct intel_link_bw_limits *old_limits,
> +				       struct intel_link_bw_limits *new_limits,
> +				       enum pipe pipe)
> +{
> +	if (pipe == INVALID_PIPE)
> +		return false;
> +
> +	if (new_limits->min_bpp_pipes & BIT(pipe))
> +		return false;
> +
> +	if (new_limits->max_bpp_x16[pipe] ==
> +	    old_limits->max_bpp_x16[pipe])
> +		return false;
> +
> +	new_limits->max_bpp_x16[pipe] =
> +		old_limits->max_bpp_x16[pipe];
> +	new_limits->min_bpp_pipes |= BIT(pipe);

So that bitmask is keeping track of pipes which have reached what
they consider to be an absolute minimum usable bpp?

The naming confused me a bit, of both the bitmask and the function.
Maybe something like 'min_bpp_reached_pipes' would be a bit more
clear, and intel_link_bw_min_bpp_reached() or something for the
function name? Still doesn't really work when just reading the
code though...

> +
> +	return true;
> +}
> +
> +static int check_all_link_config(struct intel_atomic_state *state,
> +				 struct intel_link_bw_limits *limits)
> +{
> +	/* TODO: Check all shared display link configurations like FDI */
> +	return 0;
> +}
> +
> +static bool
> +assert_link_limit_change_valid(struct drm_i915_private *i915,
> +			       const struct intel_link_bw_limits *old_limits,
> +			       const struct intel_link_bw_limits *new_limits)
> +{
> +	bool bpps_changed = false;
> +	enum pipe pipe;
> +
> +	for_each_pipe(i915, pipe) {
> +		/* The bpp limit can only decrease. */
> +		if (drm_WARN_ON(&i915->drm,
> +				new_limits->max_bpp_x16[pipe] >
> +				old_limits->max_bpp_x16[pipe]))
> +			return false;
> +
> +		if (new_limits->max_bpp_x16[pipe] <
> +		    old_limits->max_bpp_x16[pipe])
> +			bpps_changed = true;
> +	}
> +
> +	/* At least one limit must change. */
> +	if (drm_WARN_ON(&i915->drm,
> +			!bpps_changed))
> +		return false;
> +
> +	return true;
> +}
> +
> +/**
> + * intel_link_bw_atomic_check - check display link states and set a fallback config if needed
> + * @state: atomic state
> + * @new_limits: link BW limits
> + *
> + * Check the configuration of all shared display links in @state and set new BW
> + * limits in @new_limits if there is a BW limitation.
> + *
> + * Returns:
> + *   - 0 if the confugration is valid
> + *   - %-EAGAIN, if the configuration is invalid and @new_limits got updated
> + *     with fallback values with which the configuration of all CRTCs
> + *     in @state must be recomputed
> + *   - Other negative error, if the configuration is invalid without a
> + *     fallback possibility, or the check failed for another reason
> + */
> +int intel_link_bw_atomic_check(struct intel_atomic_state *state,
> +			       struct intel_link_bw_limits *new_limits)
> +{
> +	struct drm_i915_private *i915 = to_i915(state->base.dev);
> +	struct intel_link_bw_limits old_limits = *new_limits;
> +	int ret;
> +
> +	ret = check_all_link_config(state, new_limits);
> +	if (ret != -EAGAIN)
> +		return ret;
> +
> +	if (!assert_link_limit_change_valid(i915, &old_limits, new_limits))
> +		return -EINVAL;
> +
> +	return -EAGAIN;
> +}
> diff --git a/drivers/gpu/drm/i915/display/intel_link_bw.h b/drivers/gpu/drm/i915/display/intel_link_bw.h
> new file mode 100644
> index 0000000000000..e514caff5898e
> --- /dev/null
> +++ b/drivers/gpu/drm/i915/display/intel_link_bw.h
> @@ -0,0 +1,38 @@
> +/* SPDX-License-Identifier: MIT */
> +/*
> + * Copyright © 2023 Intel Corporation
> + */
> +
> +#ifndef __INTEL_LINK_BW_H__
> +#define __INTEL_LINK_BW_H__
> +
> +#include <linux/types.h>
> +
> +#include "intel_display_limits.h"
> +
> +struct drm_i915_private;
> +
> +struct intel_atomic_state;
> +struct intel_crtc_state;
> +
> +struct intel_link_bw_limits {
> +	u8 min_bpp_pipes;
> +	/* in 1/16 bpp units */
> +	int max_bpp_x16[I915_MAX_PIPES];
> +};
> +
> +void intel_link_bw_init_limits(struct drm_i915_private *i915,
> +			       struct intel_link_bw_limits *limits);
> +bool intel_link_bw_compute_pipe_bpp(struct intel_crtc_state *crtc_state);
> +int intel_link_bw_reduce_bpp(struct intel_atomic_state *state,
> +			     struct intel_link_bw_limits *limits,
> +			     u8 pipe_mask,
> +			     const char *reason);
> +bool intel_link_bw_reset_pipe_limit_to_min(struct intel_atomic_state *state,
> +					   const struct intel_link_bw_limits *old_limits,
> +					   struct intel_link_bw_limits *new_limits,
> +					   enum pipe pipe);
> +int intel_link_bw_atomic_check(struct intel_atomic_state *state,
> +			       struct intel_link_bw_limits *new_limits);
> +
> +#endif
> -- 
> 2.37.2

-- 
Ville Syrjälä
Intel