[Intel-gfx] [PATCH v6 1/2] drm/i915/gen9: Fix PCODE polling during CDCLK change notification

Thu Dec 8 14:02:52 UTC 2016

On Mon, Dec 05, 2016 at 06:27:37PM +0200, Imre Deak wrote:
> commit 848496e5902833600f7992f4faa82dc1546051ba
> Author: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Date:   Wed Jul 13 16:32:03 2016 +0300
> 
>     drm/i915: Wait up to 3ms for the pcu to ack the cdclk change request on SKL
> 
> increased the timeout to match the spec, but we still see a timeout on
> at least one SKL. A CDCLK change request following the failed one will
> succeed nevertheless.
> 
> I could reproduce this problem easily by running kms_pipe_crc_basic in a
> loop. In all failure cases _wait_for() was pre-empted for >3ms and so in
> the worst case - when the pre-emption happened right after calculating
> timeout__ in _wait_for() - we called skl_cdclk_wait_for_pcu_ready() only
> once which failed and so _wait_for() timed out. As opposed to this the
> spec says to keep retrying the request for at most a 3ms period.
> 
> To fix this send the first request explicitly to guarantee that there is
> 3ms between the first and last request. Though this matches the spec, I
> noticed that in rare cases this can still time out if we sent only a few
> requests (in the worst case 2) _and_ PCODE is busy for some reason even
> after a previous request and a 3ms delay. To work around this retry the
> polling with pre-emption disabled to maximize the number of requests.
> Also increase the timeout to 10ms to account for interrupts that could
> reduce the number of requests. With this change I couldn't trigger
> the problem.
> 
> v2:
> - Use 1ms poll period instead of 10us. (Chris)
> v3:
> - Poll with pre-emption disabled to increase the number of request
>   attempts. (Ville, Chris)
> - Factor out a helper to poll, it's also needed by the next patch.
> v4:
> - Pass reply_mask, reply to skl_pcode_request(), instead of assuming the
>   reply is generic. (Ville)
> v5:
> - List the request specific timeout values as code comment. (Ville)
> v6:
> - Try the poll first with preemption enabled.
> - Add code comment about first request being queued by PCODE. (Art)
> - Add timeout_base_ms argument. (Ville)
> 
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Art Runyan <arthur.j.runyan at intel.com>
> Reference: https://bugs.freedesktop.org/show_bug.cgi?id=97929
> Testcase: igt/kms_pipe_crc_basic/suspend-read-crc-pipe-B
> Signed-off-by: Imre Deak <imre.deak at intel.com>

The only issue I have is that buried within snb_pcode_read() is another
wait_for, now in an atomic section and we have been trying to erradicate
those. It should be happy enough, just a pita to fix later!

Other than that and a minor nit, it looks like a reasonable compromise.
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

>  drivers/gpu/drm/i915/i915_drv.h      |  2 +
>  drivers/gpu/drm/i915/intel_display.c | 31 +++++----------
>  drivers/gpu/drm/i915/intel_pm.c      | 75 ++++++++++++++++++++++++++++++++++++
>  3 files changed, 87 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index ca9786c..a2462bf 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -3597,6 +3597,8 @@ extern void intel_display_print_error_state(struct drm_i915_error_state_buf *e,
>  
>  int sandybridge_pcode_read(struct drm_i915_private *dev_priv, u32 mbox, u32 *val);
>  int sandybridge_pcode_write(struct drm_i915_private *dev_priv, u32 mbox, u32 val);
> +int skl_pcode_request(struct drm_i915_private *dev_priv, u32 mbox, u32 request,
> +		      u32 reply_mask, u32 reply, int timeout_base_ms);
>  
>  /* intel_sideband.c */
>  u32 vlv_punit_read(struct drm_i915_private *dev_priv, u32 addr);
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 1fafcce..4ef7392 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -6280,35 +6280,24 @@ skl_dpll0_disable(struct drm_i915_private *dev_priv)
>  	dev_priv->cdclk_pll.vco = 0;
>  }
>  
> -static bool skl_cdclk_pcu_ready(struct drm_i915_private *dev_priv)
> -{
> -	int ret;
> -	u32 val;
> -
> -	/* inform PCU we want to change CDCLK */
> -	val = SKL_CDCLK_PREPARE_FOR_CHANGE;
> -	mutex_lock(&dev_priv->rps.hw_lock);
> -	ret = sandybridge_pcode_read(dev_priv, SKL_PCODE_CDCLK_CONTROL, &val);
> -	mutex_unlock(&dev_priv->rps.hw_lock);
> -
> -	return ret == 0 && (val & SKL_CDCLK_READY_FOR_CHANGE);
> -}
> -
> -static bool skl_cdclk_wait_for_pcu_ready(struct drm_i915_private *dev_priv)
> -{
> -	return _wait_for(skl_cdclk_pcu_ready(dev_priv), 3000, 10) == 0;
> -}
> -
>  static void skl_set_cdclk(struct drm_i915_private *dev_priv, int cdclk, int vco)
>  {
>  	u32 freq_select, pcu_ack;
> +	int ret;
>  
>  	WARN_ON((cdclk == 24000) != (vco == 0));
>  
>  	DRM_DEBUG_DRIVER("Changing CDCLK to %d kHz (VCO %d kHz)\n", cdclk, vco);
>  
> -	if (!skl_cdclk_wait_for_pcu_ready(dev_priv)) {
> -		DRM_ERROR("failed to inform PCU about cdclk change\n");
> +	mutex_lock(&dev_priv->rps.hw_lock);
> +	ret = skl_pcode_request(dev_priv, SKL_PCODE_CDCLK_CONTROL,
> +				SKL_CDCLK_PREPARE_FOR_CHANGE,
> +				SKL_CDCLK_READY_FOR_CHANGE,
> +				SKL_CDCLK_READY_FOR_CHANGE, 3);
> +	mutex_unlock(&dev_priv->rps.hw_lock);
> +	if (ret) {
> +		DRM_ERROR("Failed to inform PCU about cdclk change (%d)\n",
> +			  ret);
>  		return;
>  	}
>  
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index 59a88de..6c2fa34 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -7864,6 +7864,81 @@ int sandybridge_pcode_write(struct drm_i915_private *dev_priv,
>  	return 0;
>  }
>  
> +static bool skl_pcode_try_request(struct drm_i915_private *dev_priv, u32 mbox,
> +				  u32 request, u32 reply_mask, u32 reply,
> +				  u32 *status)
> +{
> +	u32 val = request;
> +
> +	*status = sandybridge_pcode_read(dev_priv, mbox, &val);
> +
> +	return *status || ((val & reply_mask) == reply);
> +}
> +
> +/**
> + * skl_pcode_request - send PCODE request until acknowledgment
> + * @dev_priv: device private
> + * @mbox: PCODE mailbox ID the request is targeted for
> + * @request: request ID
> + * @reply_mask: mask used to check for request acknowledgment
> + * @reply: value used to check for request acknowledgement
> + * @timeout_base_ms: timeout for polling with preemption enabled
> + *
> + * Keep resending the @request to @mbox until PCODE acknowledges it, PCODE
> + * reports an error or an overall timeout of @timeout_base_ms+10 ms expires.
> + * The request is acknowledged once the PCODE reply dword equals @reply after
> + * applying @reply_mask. Polling is first attempted with preemption enabled
> + * for @timeout_base_ms and if this times out for another 10 ms with
> + * preemption disabled.
> + *
> + * Returns 0 on success, %-ETIMEDOUT in case of a timeout, <0 in case of some
> + * other error as reported by PCODE.
> + */
> +int skl_pcode_request(struct drm_i915_private *dev_priv, u32 mbox, u32 request,
> +		      u32 reply_mask, u32 reply, int timeout_base_ms)
> +{
> +	u32 status;
> +	int ret;
> +
> +	WARN_ON(!mutex_is_locked(&dev_priv->rps.hw_lock));
> +
> +#define COND skl_pcode_try_request(dev_priv, mbox, request, reply_mask, reply, \
> +				   &status)
> +
> +	/*
> +	 * The first request is queued by PCODE, which normally guarantees
> +	 * that a subsequent request at most @timeout_base_ms later succeeds.
> +	 * _wait_for() doesn't guarantee when its passed condition is evaluated
> +	 * first, so send the first request explicitly.
> +	 */

Scratching my head here.

/* Prime the PCODE by doing a request first. Normally it guarantees that
 * a subsequent request, at most @timeout_base_ms later, succeeds.
 */

As what I think you mean is that given preemption wait_for() doesn't
guarantee that at least two CONDs are executed. Which is reasonable, so
we want to write the code to look for a reply within timeout.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre