[Intel-gfx] [PATCH] drm/i915: Add delay on DPCD reads

[Mika Kahola]

On Tue, 2016-03-15 at 14:26 +0100, Daniel Vetter wrote:
> On Tue, Mar 15, 2016 at 01:38:58PM +0200, Mika Kahola wrote:
> > Additional 50 ms delay is needed between DPCD reads on HP Bizlink 1326
> > DP to VGA adapter. Having said that, I haven't noticed a need for
> > additional delay between DPCD reads on other DP-VGA dongles.
> > 
> > While at it, let's replace mdelay() with usleep_range() routine.
> > 
> > Signed-off-by: Mika Kahola <mika.kahola at intel.com>
> 
> Please don't add more hacks to our own private copy of read_wake. This
> stuff needs to move into drm core, since really we can't be the only ones
> who try to plug in shit dongles ;-)
> 
> Would be great to test this with an amd DP card and the exact same
> connector and see what happens. Then move these read_wake hacks into the
> drm_dp_dpcd_read function. Lyude is also working on something like this,
> but for DP MST hubs.

This particular DP-VGA dongle has been problematic. I have used Windows
to benchmark the performance. It takes eternity to light up the display
on Windows but the eventually the driver is able to do that.

You're right, perhaps the best place for this sort of hacks is the drm
core. 

Cheers,
Mika
 
> -Daniel
> 
> > ---
> >  drivers/gpu/drm/i915/intel_dp.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index 0e326e7..ff3883c 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -3125,9 +3125,11 @@ intel_dp_dpcd_read_wake(struct drm_dp_aux *aux, unsigned int offset,
> >  
> >  	for (i = 0; i < 3; i++) {
> >  		ret = drm_dp_dpcd_read(aux, offset, buffer, size);
> > -		if (ret == size)
> > +		if (ret == size) {
> > +			usleep_range(50000, 50100);
> >  			return ret;
> > -		msleep(1);
> > +		}
> > +		usleep_range(1000, 1100);
> >  	}
> >  
> >  	return ret;
> > -- 
> > 1.9.1
> > 
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>