Re: [PATCH 3/3] drm/i915: go back to switch for VLV mem freq detection

[Ben Widawsky <ben@bwidawsk.net>]

On Wed, May 01, 2013 at 06:38:17PM -0700, Kenneth Graunke wrote:
> On 05/01/2013 06:26 PM, Ben Widawsky wrote:
> >On Wed, May 01, 2013 at 04:09:22PM -0700, Jesse Barnes wrote:
> >>We need to catch the invalid case and override it.
> >>
> >>Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
> 
> Jesse, it's actually worse than that:
> 
> Your parenthesis were off in the previous version, causing it to do
> the multiplication by 266 before the bitwise-and:
> 
>    800 + ((266 * (val >> 6)) & 3)
> 
> This meant that you always got 800 or 802 Mhz.
> 
> Also, even when corrected, your previous formula would return 1332
> Mhz instead of 1333 Mhz, which is slightly off and doesn't work
> since you use it in explicit switch statements.
> 
> It might be worth mentioning in the commit message.
> 
> >>---
> >>  drivers/gpu/drm/i915/intel_pm.c |   16 +++++++++++++++-
> >>  1 file changed, 15 insertions(+), 1 deletion(-)
> >>
> >>diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> >>index 0f4b46e..0a3b0b3 100644
> >>--- a/drivers/gpu/drm/i915/intel_pm.c
> >>+++ b/drivers/gpu/drm/i915/intel_pm.c
> >>@@ -2902,7 +2902,21 @@ static void valleyview_enable_rps(struct drm_device *dev)
> >>  		   GEN7_RC_CTL_TO_MODE);
> >>
> >>  	valleyview_punit_read(dev_priv, PUNIT_REG_GPU_FREQ_STS, &val);
> >>-	dev_priv->mem_freq = 800 + (266 * (val >> 6) & 3);
> >>+	switch ((val >> 6) & 3) {
> >>+	case 0:
> >>+		dev_priv->mem_freq = 800;
> >>+		break;
> >>+	case 1:
> >>+		dev_priv->mem_freq = 1066;
> >>+		break;
> >>+	case 2:
> >>+		dev_priv->mem_freq = 1333;
> >>+		break;
> >>+	case 3:
> >>+		DRM_ERROR("invalid mem freq, assuming 800MHz\n");
> >>+		dev_priv->mem_freq = 800;
> >>+		break;
> >>+	}
> >>  	DRM_DEBUG_DRIVER("DDR speed: %d MHz", dev_priv->mem_freq);
> >>
> >>  	DRM_DEBUG_DRIVER("GPLL enabled? %s\n", val & 0x10 ? "yes" : "no");
> >
> >
> >I guess it doesn't handle the last case, but:
> >dev_priv->mem_freq = 800 + (266 * (val))) + val/2
> 
> Don't use this method.  Presumably Ben meant (val >> 6) here, but
> even with that fix this formula returns 1067 rather than 1066, which
> is wrong.

You're right about the missing shift, but otherwise the formula is
correct. Val is 1 in the case of 1066MHz...

> 
> >or
> >
> >u32 freqs[] = {800,1066,1333,800};
> >dev_priv->mem_freq = freqs[(val >> 6) & 3];
> 
> I like this table based approach best.  It's much more succinct than
> the switch statement, and exact.  Might be worth preserving the
> DRM_ERROR.
> 
> >Are two ways I would have used before making a switch block :P
> >
> >Just a thought, but perhaps we don't want to enable RPS if we can't
> >reliably figure out the memory freq. (case 3)
> >
> >Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
> 
> For either the switch or Ben's table:
> Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
> 
> Incidentally, my Punit returns 3 here, but I believe the memory is
> actually 1066.  Not sure what the deal is.
> 
> --Ken
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-- 
Ben Widawsky, Intel Open Source Technology Center