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

From: Ben Skeggs
Date: Thu Jul 10 2014 - 22:01:20 EST

On Fri, Jul 11, 2014 at 11:49 AM, Alexandre Courbot <acourbot@xxxxxxxxxx> 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.

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