Re: [Intel-gfx] [PATCH 1/2] drm/i915/display: Restore dsparb_lock.

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Mar 08, 2023 at 11:58:58AM -0500, Rodrigo Vivi wrote:
> uncore->lock only protects the forcewake domain itself,
> not the register accesses.
> 
> uncore's _fw alternatives are for cases where the domains
> are not needed because we are sure that they are already
> awake.
> 
> So the move towards the uncore's _fw alternatives seems
> right, however using the uncore-lock to protect the dsparb
> registers seems an abuse of the uncore-lock.
> 
> Let's restore the previous individual lock and try to get
> rid of the direct uncore accesses from the display code.
> 
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> ---
>  drivers/gpu/drm/i915/display/i9xx_wm.c            | 13 ++-----------
>  drivers/gpu/drm/i915/display/intel_display_core.h |  3 +++
>  drivers/gpu/drm/i915/i915_driver.c                |  1 +
>  3 files changed, 6 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/i9xx_wm.c b/drivers/gpu/drm/i915/display/i9xx_wm.c
> index caef72d38798..8fe0b5c63d3a 100644
> --- a/drivers/gpu/drm/i915/display/i9xx_wm.c
> +++ b/drivers/gpu/drm/i915/display/i9xx_wm.c
> @@ -1771,16 +1771,7 @@ static void vlv_atomic_update_fifo(struct intel_atomic_state *state,
>  
>  	trace_vlv_fifo_size(crtc, sprite0_start, sprite1_start, fifo_size);
>  
> -	/*
> -	 * uncore.lock serves a double purpose here. It allows us to
> -	 * use the less expensive I915_{READ,WRITE}_FW() functions, and
> -	 * it protects the DSPARB registers from getting clobbered by
> -	 * parallel updates from multiple pipes.
> -	 *
> -	 * intel_pipe_update_start() has already disabled interrupts
> -	 * for us, so a plain spin_lock() is sufficient here.
> -	 */

I was wondering if we need to preserve the comment about irqs,
but since this is the only place using this lock, and it's never
called from an irq handler a non-irq disabling spinlock will suffice
anyway.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> -	spin_lock(&uncore->lock);
> +	spin_lock(&dev_priv->display.wm.dsparb_lock);
>  
>  	switch (crtc->pipe) {
>  	case PIPE_A:
> @@ -1840,7 +1831,7 @@ static void vlv_atomic_update_fifo(struct intel_atomic_state *state,
>  
>  	intel_uncore_posting_read_fw(uncore, DSPARB);
>  
> -	spin_unlock(&uncore->lock);
> +	spin_unlock(&dev_priv->display.wm.dsparb_lock);
>  }
>  
>  #undef VLV_FIFO
> diff --git a/drivers/gpu/drm/i915/display/intel_display_core.h b/drivers/gpu/drm/i915/display/intel_display_core.h
> index fdab7bb93a7d..68c6bfb91dbe 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_core.h
> +++ b/drivers/gpu/drm/i915/display/intel_display_core.h
> @@ -253,6 +253,9 @@ struct intel_wm {
>  	 */
>  	struct mutex wm_mutex;
>  
> +	/* protects DSPARB registers on pre-g4x/vlv/chv */
> +	spinlock_t dsparb_lock;
> +
>  	bool ipc_enabled;
>  };
>  
> diff --git a/drivers/gpu/drm/i915/i915_driver.c b/drivers/gpu/drm/i915/i915_driver.c
> index a53fd339e2cc..c78e36444a12 100644
> --- a/drivers/gpu/drm/i915/i915_driver.c
> +++ b/drivers/gpu/drm/i915/i915_driver.c
> @@ -223,6 +223,7 @@ static int i915_driver_early_probe(struct drm_i915_private *dev_priv)
>  	mutex_init(&dev_priv->display.pps.mutex);
>  	mutex_init(&dev_priv->display.hdcp.comp_mutex);
>  	spin_lock_init(&dev_priv->display.dkl.phy_lock);
> +	spin_lock_init(&dev_priv->display.wm.dsparb_lock);
>  
>  	i915_memcpy_init_early(dev_priv);
>  	intel_runtime_pm_init_early(&dev_priv->runtime_pm);
> -- 
> 2.39.2

-- 
Ville Syrjälä
Intel