linux-kernel - Re: [PATCH 0/3] drm/gk20a: support for reclocking

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Message-ID: <20140711105647.GA23218@tbergstrom-lnx.Nvidia.com>
Date:	Fri, 11 Jul 2014 13:56:47 +0300
From:	Peter De Schrijver <pdeschrijver@...dia.com>
To:	Ben Skeggs <skeggsb@...il.com>
CC:	Alex Courbot <acourbot@...dia.com>,
	"gnurou@...il.com" <gnurou@...il.com>,
	"nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"Ben Skeggs" <bskeggs@...hat.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH 0/3] drm/gk20a: support for reclocking

On Fri, Jul 11, 2014 at 04:01:02AM +0200, Ben Skeggs wrote:
> On Fri, Jul 11, 2014 at 11:49 AM, Alexandre Courbot <acourbot@...dia.com> wrote:
> > On 07/10/2014 06:43 PM, Peter De Schrijver wrote:
> >>
> >> On Thu, Jul 10, 2014 at 09:34:34AM +0200, Alexandre Courbot wrote:
> >>>
> >>> This series adds support for reclocking on GK20A. The first two patches
> >>> touch
> >>> the clock subsystem to allow GK20A to operate, by making the presence of
> >>> the
> >>> thermal and voltage devices optional, and allowing pstates to be provided
> >>> directly instead of being probed using the BIOS (which Tegra does not
> >>> have).
> >>>
> >>> The last patch adds the GK20A clock device. Arguably the clock can be
> >>> seen as a
> >>> stripped-down version of what is seen on NVE0, however instead of using
> >>> NVE0
> >>> support has been written from scratch using the ChromeOS kernel as a
> >>> basis.
> >>> There are several reasons for this:
> >>>
> >>> - The ChromeOS driver uses a lookup table for the P coefficient which I
> >>> could
> >>>    not find in the NVE0 driver,
> >>> - Some registers that NVE0 expects to find are not present on GK20A (e.g.
> >>>    0x137120 and 0x137140),
> >>> - Calculation of MNP is done differently from what is performed in
> >>>    nva3_pll_calc(), and it might be interesting to compare the two
> >>> methods,
> >>> - All the same, the programming sequence is done differently in the
> >>> ChromeOS
> >>>    driver and NVE0 could possibly benefit from it (?)
> >>>
> >>> It would be interesting to try and merge both, but for now I prefer to
> >>> have the
> >>> two coexisting to ensure proper operation on GK20A and besure I don't
> >>> break
> >>> dGPU support. :)
> >>>
> >>> Regarding the first patch, one might argue that I could as well add
> >>> thermal
> >>> and voltage devices to GK20A. The reason this is not done is because
> >>> these
> >>> currently depend heavily on the presence of a BIOS, and will require a
> >>> rework
> >>> similar to that done in patch 2 for clocks. I would like to make sure
> >>> this
> >>> approach is approved because applying it to other subdevs.
> >>
> >>
> >> I think this should use CCF so we can use pre and post rate change
> >> notifiers
> >> to hookup vdd_gpu DVS.
> >
> >
> > Do you mean that we should turn the Nouveau gk20a clock driver into a
> > consumer of this CCF clock? I have nothing against this, but note that
> > Nouveau can also perform DVS on its own, as the pstates can also contain a
> > voltage to be applied to the volt device (not yet implemented in this
> > series).
> >
> > The question then becomes whether we want an additional layer of abstraction
> > on these devices and whether the pre/post rate change notifiers give us any
> > advantage compared to what Nouveau currently proposes.
> I had a brief look at this, and personally I don't think the CCF is a
> very good match at all for how we're *supposed* to manage clock
> frequencies as described by a discrete GPU VBIOS, and especially for
> when we get to the point of using the PMU falcon to coordinate all the
> various bits and pieces that go towards power management.
> 

For all I can see, the PMU is not involved in the mechanics of GPU frequency
scaling on Tegra.

Cheers,

Peter.
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