[Intel-gfx] [PATCH] drm/i915: Limit the watermark to at least 8 entries on gen2/3

[Ville Syrjälä]

On Fri, Sep 05, 2014 at 09:03:11PM +0200, Thomas Richter wrote:
> On 05.09.2014 20:54, ville.syrjala at linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> >
> > 830 is very unhappy of the watermark value is too low (indicating a very
> > high watermark in fact, ie. memory fetch will occur with an almost full
> > FIFO). Limit the watermark value to at least 8 cache lines.
> >
> > That also matches the burst size we use on most platforms. BSpec seems
> > to indicate we should limit the watermark to 'burst size + 1'. But on
> > gen4 we already use a hardcoded 8 as the watermark value (as the spec
> > says we should), so just use 8 as the limit on gen2/3 as well.
> 
> /* snip */
> 
> That's certainly one way to fix it. Nothing against it. It might be more 
> conservative to make this fix only in the calling context of the 830 and 
> 845 specific functions as I don't know whether the other chipsets have 
> similar issues. I.e. one level up in the call-chain.
> 
> This being said, I can certainly confirm that the suggested modification 
> works on i830.
> 
> One way or another, it seems to me that drm-intel-nightly which I pulled 
> had a couple of more serious issues. I didn't even get X running, and 
> monitor detection did not seem to work reliable, too. I had the same 
> kind of instability on the external monitor for the console Ville's 
> patches fixed so nicely, so it seems to me that there is something else 
> that is missing. The new DVO code was definitely there, I checked that.

The DVO 2x clock fix was missing. I posted a new version. But I think
that should only affect the DVO output and not the VGA output. Anyway on
my 830 nightly with those two patches is pretty damn good. I'm going to
assume this is the first time in history when 830 works properly in linux
(well, excluding all the features we still haven't even implemented in
the driver). Only took 14 years or so to get there :P

-- 
Ville Syrjälä
Intel OTC