[Intel-gfx] [PATCH 2/3] drm/i915: Introduce dual_bsd_ring parameter.

[Ben Widawsky]

On Mon, Jun 30, 2014 at 09:51:10AM -0700, Rodrigo Vivi wrote:
> On Broadwell GT3 we have 2 Video Command Streamers (VCS),
> but userspace has no control when using VCS1 or VCS2. So we cannot test,
> validate or debug specific changes or workaround that might affect only
> one or another ring. So this patch introduces a mechanism to avoid the
> ping-pong selection and use one specific ring given at boot time.
> 
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
I think going the debugfs route is the right thing to do if this should
move forward. If the goal is only for test, then I'm not really in favor
of moving it forward.

> ---
>  drivers/gpu/drm/i915/i915_drv.h            |  1 +
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c | 34 ++++++++++++++++++------------
>  drivers/gpu/drm/i915/i915_params.c         |  6 ++++++
>  3 files changed, 27 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 8cea596..7b6614f 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2069,6 +2069,7 @@ struct i915_params {
>  	int panel_ignore_lid;
>  	unsigned int powersave;
>  	int semaphores;
> +	int dual_bsd_ring;
>  	unsigned int lvds_downclock;
>  	int lvds_channel_mode;
>  	int panel_use_ssc;
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index d815ef5..09f350e 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -1035,26 +1035,32 @@ static int gen8_dispatch_bsd_ring(struct drm_device *dev,
>  {
>  	struct drm_i915_private *dev_priv = dev->dev_private;
>  	struct drm_i915_file_private *file_priv = file->driver_priv;
> +	int ring_id;
> +	int dual = i915.dual_bsd_ring;
>  
>  	/* Check whether the file_priv is using one ring */
>  	if (file_priv->bsd_ring)
>  		return file_priv->bsd_ring->id;
> -	else {
> -		/* If no, use the ping-pong mechanism to select one ring */
> -		int ring_id;
>  
> -		mutex_lock(&dev->struct_mutex);
> -		if (dev_priv->mm.bsd_ring_dispatch_index == 0) {
> -			ring_id = VCS;
> -			dev_priv->mm.bsd_ring_dispatch_index = 1;
> -		} else {
> -			ring_id = VCS2;
> -			dev_priv->mm.bsd_ring_dispatch_index = 0;
> -		}
> -		file_priv->bsd_ring = &dev_priv->ring[ring_id];
> -		mutex_unlock(&dev->struct_mutex);
> -		return ring_id;
> +	/* If no, use the parameter defined or ping-pong mechanism
> +	 * to select one ring */
> +	mutex_lock(&dev->struct_mutex);
> +
> +	if (dual == 1 || (dual != 2 &&
> +			  dev_priv->mm.bsd_ring_dispatch_index == 0)) {
> +		ring_id = VCS;
> +		dev_priv->mm.bsd_ring_dispatch_index = 1;
> +	} else {
> +		ring_id = VCS2;
> +		dev_priv->mm.bsd_ring_dispatch_index = 0;
>  	}
> +
> +	file_priv->bsd_ring = &dev_priv->ring[ring_id];
> +	mutex_unlock(&dev->struct_mutex);
> +
> +	WARN(dual, "Forcibly trying to use only one bsd ring. Using: %s\n",
> +	     file_priv->bsd_ring->name);

Don't warn if this is valid module parameter people can use.

> +	return ring_id;
>  }
>  
>  static struct drm_i915_gem_object *
> diff --git a/drivers/gpu/drm/i915/i915_params.c b/drivers/gpu/drm/i915/i915_params.c
> index 8145729..d4871c8 100644
> --- a/drivers/gpu/drm/i915/i915_params.c
> +++ b/drivers/gpu/drm/i915/i915_params.c
> @@ -29,6 +29,7 @@ struct i915_params i915 __read_mostly = {
>  	.panel_ignore_lid = 1,
>  	.powersave = 1,
>  	.semaphores = -1,
> +	.dual_bsd_ring = 0,
>  	.lvds_downclock = 0,
>  	.lvds_channel_mode = 0,
>  	.panel_use_ssc = -1,
> @@ -70,6 +71,11 @@ MODULE_PARM_DESC(semaphores,
>  	"Use semaphores for inter-ring sync "
>  	"(default: -1 (use per-chip defaults))");
>  
> +module_param_named(dual_bsd_ring, i915.dual_bsd_ring, int, 0600);
> +MODULE_PARM_DESC(dual_bsd_ring,
> +	 "Specify bds rings for VCS when there are multiple VCSs available."
> +	 "(0=All available bsd rings [default], 1=only VCS1, 2=only VCS2)");
> +

It looks like life would be a lot easier if you did
0 = VCS
1 = VCS2
2 = BOTH

>  module_param_named(enable_rc6, i915.enable_rc6, int, 0400);
>  MODULE_PARM_DESC(enable_rc6,
>  	"Enable power-saving render C-state 6. "

Can't spot anything wrong, but again, I don't think the patch is worth
merging unless we can find a good use case.

-- 
Ben Widawsky, Intel Open Source Technology Center