Re: [PATCH v4 2/3] drm/i915/gen9: Fix PCODE polling during CDCLK change notification

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On Mon, Nov 28, 2016 at 06:12:51PM +0200, Imre Deak wrote:
> On ma, 2016-11-28 at 17:58 +0200, Ville Syrjälä wrote:
> > On Mon, Nov 28, 2016 at 05:29:28PM +0200, Imre Deak wrote:
> > > commit 848496e5902833600f7992f4faa82dc1546051ba
> > > Author: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> > > 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 disable pre-emption to maximize the number of times we retry
> > > the request. Also increase the timeout to 10ms to account for interrupts
> > > that could reduce the number of these attempts. 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)
> > > 
> > > Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> > > Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > > 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@xxxxxxxxx>
> > > ---
> > >  drivers/gpu/drm/i915/i915_drv.h      |  2 ++
> > >  drivers/gpu/drm/i915/intel_display.c | 31 ++++++++---------------
> > >  drivers/gpu/drm/i915/intel_pm.c      | 49 ++++++++++++++++++++++++++++++++++++
> > >  3 files changed, 61 insertions(+), 21 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> > > index 01f5067..1be5bab 100644
> > > --- a/drivers/gpu/drm/i915/i915_drv.h
> > > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > > @@ -3593,6 +3593,8 @@ 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);
> > >  
> > >  /* 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 5d11002..3d220da 100644
> > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > @@ -6245,35 +6245,24 @@ 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);
> > > +	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 66c62f3..aed88e0 100644
> > > --- a/drivers/gpu/drm/i915/intel_pm.c
> > > +++ b/drivers/gpu/drm/i915/intel_pm.c
> > > @@ -7864,6 +7864,55 @@ 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
> > > + *
> > > + * Keep resending the @request to @mbox until PCODE acknowledges it, PCODE
> > > + * reports an error or a 10ms timeout expires. The request is acknowledged
> > > + * once the PCODE reply dword equals @reply after aplying @reply_mask.
> > > + *
> > > + * 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)
> > > +{
> > > +	u32 status;
> > > +	int ret;
> > > +
> > > +	WARN_ON(!mutex_is_locked(&dev_priv->rps.hw_lock));
> > > +
> > > +	/*
> > > +	 * The spec says to keep retrying the request for at most 3ms until
> > 
> > This comment seems a little misplaced. The 3ms is specified for the
> > cdclk prep request, SAGV will have a different value.
> > If we don't want callers to pass in the timeout as well, I guess we'll
> > at least want to reword this comment to say which callers need what
> > kind of timeouts.
> 
> How about "at most 3ms - which can be less depending on the given
> request"?

I'd rather want explicit "cdclk thing needs 3ms, sagv thing needs
1ms" (or whatever the sagv timeout was). Otherwise it's hard to see
whether it's even doing the correct thing.

> 
> > Though I kinda like the idea of callers passing that
> > in as well.
> 
> The 10ms timeout I chose here is anyway an upper bound, considering
> that interrupts could reduce the number of requests we can send. Hence
> thought to make it a generic value.
> 
> > 
> > > +	 * acknowledgement, so disable pre-emption to maximize the number of
> > > +	 * attempts within this duration. Use a 10ms overall timeout to
> > > +	 * account for interrupts that could reduce the number of attempts.
> > > +	 */
> > > +	preempt_disable();
> > > +	ret = wait_for_atomic(skl_pcode_try_request(dev_priv, mbox, request,
> > > +						    reply_mask, reply, &status),
> > > +			      10);
> > > +	preempt_enable();
> > > +
> > > +	return ret ? ret : status;
> > > +}
> > > +
> > >  static int byt_gpu_freq(struct drm_i915_private *dev_priv, int val)
> > >  {
> > >  	/*
> > > -- 
> > > 2.5.0
> > 

-- 
Ville Syrjälä
Intel OTC
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