Re: [PATCH 13/21] drm/dp: Do not busy-loop during link training

From: Thierry Reding <thierry.reding@gmail.com>
To: Philipp Zabel <p.zabel@pengutronix.de>
Cc: dri-devel@lists.freedesktop.org
Subject: Re: [PATCH 13/21] drm/dp: Do not busy-loop during link training
Date: Mon, 2 Sep 2019 12:57:36 +0200	[thread overview]
Message-ID: <20190902105736.GA19263@ulmo> (raw)
In-Reply-To: <1565271207.3656.17.camel@pengutronix.de>

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On Thu, Aug 08, 2019 at 03:33:27PM +0200, Philipp Zabel wrote:
> On Mon, 2019-08-05 at 14:23 +0200, Thierry Reding wrote:
> > From: Thierry Reding <treding@nvidia.com>
> > 
> > Use microsecond sleeps for the clock recovery and channel equalization
> > delays during link training. The duration of these delays can be from
> > 100 us up to 16 ms. It is rude to busy-loop for that amount of time.
> > 
> > While at it, also convert to standard coding style by putting the
> > opening braces in a function definition on a new line.
> > 
> > Signed-off-by: Thierry Reding <treding@nvidia.com>
> > ---
> >  drivers/gpu/drm/drm_dp_helper.c | 30 ++++++++++++++++++------------
> >  1 file changed, 18 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> > index 4112570dbe67..344f887fef2c 100644
> > --- a/drivers/gpu/drm/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/drm_dp_helper.c
> > @@ -120,33 +120,39 @@ u8 drm_dp_get_adjust_request_pre_emphasis(const u8 link_status[DP_LINK_STATUS_SI
> >  }
> >  EXPORT_SYMBOL(drm_dp_get_adjust_request_pre_emphasis);
> >  
> > -void drm_dp_link_train_clock_recovery_delay(const u8 dpcd[DP_RECEIVER_CAP_SIZE]) {
> > -	int rd_interval = dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
> > -			  DP_TRAINING_AUX_RD_MASK;
> > +void drm_dp_link_train_clock_recovery_delay(const u8 dpcd[DP_RECEIVER_CAP_SIZE])
> > +{
> > +	unsigned int rd_interval = dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
> > +					DP_TRAINING_AUX_RD_MASK;
> >  
> >  	if (rd_interval > 4)
> > -		DRM_DEBUG_KMS("AUX interval %d, out of range (max 4)\n",
> > +		DRM_DEBUG_KMS("AUX interval %u, out of range (max 4)\n",
> >  			      rd_interval);
> >  
> >  	if (rd_interval == 0 || dpcd[DP_DPCD_REV] >= DP_DPCD_REV_14)
> > -		udelay(100);
> > +		rd_interval = 100;
> >  	else
> > -		mdelay(rd_interval * 4);
> 
> rd_interval was in units of 4 ms.
> 
> > +		rd_interval *= 4;
> 
> This should be *= 4000 since the result is fed into usleep_range instead
> of mdelay.

Good catch! Fixed in v2.

> 
> > +
> > +	usleep_range(rd_interval, rd_interval * 2);
> >  }
> >  EXPORT_SYMBOL(drm_dp_link_train_clock_recovery_delay);
> >  
> > -void drm_dp_link_train_channel_eq_delay(const u8 dpcd[DP_RECEIVER_CAP_SIZE]) {
> > -	int rd_interval = dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
> > -			  DP_TRAINING_AUX_RD_MASK;
> > +void drm_dp_link_train_channel_eq_delay(const u8 dpcd[DP_RECEIVER_CAP_SIZE])
> > +{
> > +	unsigned int rd_interval = dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
> > +					DP_TRAINING_AUX_RD_MASK;
> >  
> >  	if (rd_interval > 4)
> > -		DRM_DEBUG_KMS("AUX interval %d, out of range (max 4)\n",
> > +		DRM_DEBUG_KMS("AUX interval %u, out of range (max 4)\n",
> >  			      rd_interval);
> >  
> >  	if (rd_interval == 0)
> > -		udelay(400);
> > +		rd_interval = 400;
> >  	else
> > -		mdelay(rd_interval * 4);
> > +		rd_interval *= 4;
> 
> Same as above.

Fixed in v2.

Thanks,
Thierry

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