Re: [PATCH] drm/i915: Extend residency counter ranges on chv and byt

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Ville Syrjälä <ville.syrjala@linux.intel.com> writes:

> On Thu, Mar 02, 2017 at 06:21:37PM +0000, Chris Wilson wrote:
>> On Thu, Mar 02, 2017 at 08:13:31PM +0200, Ville Syrjälä wrote:
>> > On Thu, Mar 02, 2017 at 08:01:40PM +0200, Mika Kuoppala wrote:
>> > > We were passively acting on the high counter value bit
>> > > and as it was never set, we were only utilizing the
>> > > the 32bits of resolution. As the divisor with these platforms
>> > > is quite high, the wrap around happened in the less than 13 seconds.
>> > > 
>> > > If we toggle the resolution bit in the control register and
>> > 
>> > Can't be done on all machines. IIRC both Chris and me tried this at
>> > some point and on some machines the register was locked. Also I'm
>> > not sure if some piece of firmware depends on the original setting.
>> 
>> > Ville Syrjälä wrote:
>> > On Thu, Apr 07, 2016 at 02:58:01PM +0100, Chris Wilson wrote:
>> > > On Thu, Apr 07, 2016 at 04:18:16PM +0300, Ville Syrjälä wrote:
>> > > > On Thu, Apr 07, 2016 at 01:59:44PM +0100, Chris Wilson wrote:
>> > > > > Can we set that bit ourselves? That puts the overflow into the 1 hour
>> > > > > mark. Thanks,
>> > > > 
>> > > > I don't know if it's safe to frob the bit. I worry that something
>> > > > outside our control might depend on it staying put.
>> > > 
>> > > A quick frob of the bit says that it is RO. When I try to set it, it
>> > > doesn't stick. :(
>> > 
>> > Same here on my VLV. I was able to toggle it on my BSW. Perhaps
>> > something can lock it down, and my BSW BIOS just doesn't do that.
>> 
>> Bah. Humbug.
>
> Hmm. Actually now that I tried it again it seems to stick at least on
> one BSW and two VLV machines. I'm not 100% those VLV machines are what I
> tried before, but the third one I have has died so I can't re-test it.
>
> The other explanation for the failure I saw earlier could be that I
> forgot to set the mask bit. Interestingly the mask bit reads as 0 on
> VLV and as 1 on CHV, but when writing both need it to be set (as is
> expected for a mask bit).

I falled for this trap atleast on the first try. The bspec nomenclature
on the page was somewhat nonstandard, mask bits misleadingly 'RO'
so my brain skipped those at first read...

-Mika
>
> BTW I found this note in the Gunit docs:
> "Each of these counters will start counting on reset and continue
>  counting when that particular event happens. There will be a 40-bit
>  counter. 0x13_8104[15] selects if the lower 32 bits or the upper
>  32 bits of the 40-bit counter are read out. 0x13_8104[7:0] will
>  have individual enables for each of the above counters"
>
> Hmm. It also looks like we're explicitly enabling the high range mode on
> CHV in rps init. But on VLV we don't touch that bit. I can't see this
> register being mentioned in the BIOS spec, so I'm not sure who came up
> with this code. Oh, actually it seems we were enabling the high range
> bit on VLV too until commit 31685c258e0b ("drm/i915/vlv: WA for Turbo
> and RC6 to work together.")
>
> So all this leads to me to think that maybe it's safe to frob this
> register after all. If it were super important for turbo/rc6 I would
> have expected to see it in the BIOS spec.
>
> -- 
> Ville Syrjälä
> Intel OTC
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