[PATCH 3/3] drm/omap: Filter displays mode based on bandwidth limit

[Daniel Vetter]

On Fri, Oct 20, 2017 at 04:12:58PM +0300, Peter Ujfalusi wrote:
> If we have memory bandwidth limit configured, reject the modes which would
> require more bandwidth than the limit if it is used with one full
> resolution plane (most common use case).
> 
> This filtering is not providing full protection as it is possible that
> application would pick smaller crtc resolution with high resolution planes
> and down scaling, or can enable more smaller planes where the sum of their
> bandwidth need would be higher than the limit.

Note that userspace can also just pick a mode it wants, so you need to
have a proper bw check in your atomic_check logic anyway. There's some
recent work to sprinkle ->mode_valid checks over objects to help a bit
with filling that gap, you might want to look into a crtc->mode_valid
callback for this logic at least.
-Daniel
> 
> This patch only allows us to filter out modes which would need more
> bandwidth if they were used with one full screen plane.
> 
> Signed-off-by: Peter Ujfalusi <peter.ujfalusi at ti.com>
> ---
>  drivers/gpu/drm/omapdrm/omap_connector.c | 24 ++++++++++++++++++++++++
>  drivers/gpu/drm/omapdrm/omap_drv.c       |  5 +++++
>  drivers/gpu/drm/omapdrm/omap_drv.h       |  3 +++
>  3 files changed, 32 insertions(+)
> 
> diff --git a/drivers/gpu/drm/omapdrm/omap_connector.c b/drivers/gpu/drm/omapdrm/omap_connector.c
> index aa5ba9ae2191..c693d22960c8 100644
> --- a/drivers/gpu/drm/omapdrm/omap_connector.c
> +++ b/drivers/gpu/drm/omapdrm/omap_connector.c
> @@ -159,6 +159,7 @@ static int omap_connector_mode_valid(struct drm_connector *connector,
>  				 struct drm_display_mode *mode)
>  {
>  	struct omap_connector *omap_connector = to_omap_connector(connector);
> +	struct omap_drm_private *priv = connector->dev->dev_private;
>  	struct omap_dss_device *dssdev = omap_connector->dssdev;
>  	struct omap_dss_driver *dssdrv = dssdev->driver;
>  	struct videomode vm = {0};
> @@ -203,6 +204,29 @@ static int omap_connector_mode_valid(struct drm_connector *connector,
>  		drm_mode_destroy(dev, new_mode);
>  	}
>  
> +	/* Check for bandwidth limit */
> +	if (!r && priv->max_bandwidth) {
> +		unsigned int bandwidth, real_refresh;
> +
> +		/*
> +		 * Estimation for the bandwidth need of a given mode with one
> +		 * full screen plane:
> +		 * resolution * 32bpp * (real) refresh rate
> +		 */
> +		real_refresh = drm_mode_vrefresh(mode);
> +		if (mode->flags & DRM_MODE_FLAG_INTERLACE)
> +			real_refresh /= 2;
> +
> +		bandwidth = vm.hactive * vm.vactive * 4 * real_refresh;
> +
> +		/*
> +		 * Reject modes which would need more bandwidth if used with one
> +		 * full resolution plane (most common use case).
> +		 */
> +		if (priv->max_bandwidth < bandwidth)
> +			ret = MODE_BAD;
> +	}
> +
>  	DBG("connector: mode %s: "
>  			"%d:\"%s\" %d %d %d %d %d %d %d %d %d %d 0x%x 0x%x",
>  			(ret == MODE_OK) ? "valid" : "invalid",
> diff --git a/drivers/gpu/drm/omapdrm/omap_drv.c b/drivers/gpu/drm/omapdrm/omap_drv.c
> index 528cf5c25bec..58b4a0e149e0 100644
> --- a/drivers/gpu/drm/omapdrm/omap_drv.c
> +++ b/drivers/gpu/drm/omapdrm/omap_drv.c
> @@ -701,6 +701,11 @@ static int pdev_probe(struct platform_device *pdev)
>  	spin_lock_init(&priv->list_lock);
>  	INIT_LIST_HEAD(&priv->obj_list);
>  
> +	/* Get memory bandwidth limits */
> +	if (priv->dispc_ops->get_memory_bandwidth_limit)
> +		priv->max_bandwidth =
> +				priv->dispc_ops->get_memory_bandwidth_limit();
> +
>  	omap_gem_init(ddev);
>  
>  	ret = omap_modeset_init(ddev);
> diff --git a/drivers/gpu/drm/omapdrm/omap_drv.h b/drivers/gpu/drm/omapdrm/omap_drv.h
> index cccaae787a7c..d49715272080 100644
> --- a/drivers/gpu/drm/omapdrm/omap_drv.h
> +++ b/drivers/gpu/drm/omapdrm/omap_drv.h
> @@ -86,6 +86,9 @@ struct omap_drm_private {
>  	spinlock_t wait_lock;		/* protects the wait_list */
>  	struct list_head wait_list;	/* list of omap_irq_wait */
>  	uint32_t irq_mask;		/* enabled irqs in addition to wait_list */
> +
> +	/* memory bandwidth limit if it is needed on the platform */
> +	unsigned int max_bandwidth;
>  };
>  
>  
> -- 
> Peter
> 
> Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
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> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch