Re: [RFC v2 1/1] drm/lima: Add optional devfreq support

[Robin Murphy]

On 2019-12-31 4:47 pm, Martin Blumenstingl wrote:
> Hi Robin,
>
> On Tue, Dec 31, 2019 at 5:40 PM Robin Murphy <robin.murphy@xxxxxxx> wrote:
> > On 2019-12-31 2:17 pm, Martin Blumenstingl wrote:
> > > Hi Robin,
> > >
> > > On Mon, Dec 30, 2019 at 1:47 AM Robin Murphy <robin.murphy@xxxxxxx> wrote:
> > > > On 2019-12-29 11:19 pm, Martin Blumenstingl wrote:
> > > > > Hi Robin,
> > > > >
> > > > > On Sun, Dec 29, 2019 at 11:58 PM Robin Murphy <robin.murphy@xxxxxxx> wrote:
> > > > > > Hi Martin,
> > > > > >
> > > > > > On 2019-12-27 5:37 pm, Martin Blumenstingl wrote:
> > > > > > > Most platforms with a Mali-400 or Mali-450 GPU also have support for
> > > > > > > changing the GPU clock frequency. Add devfreq support so the GPU clock
> > > > > > > rate is updated based on the actual GPU usage when the
> > > > > > > "operating-points-v2" property is present in the board.dts.
> > > > > > >
> > > > > > > The actual devfreq code is taken from panfrost_devfreq.c and modified so
> > > > > > > it matches what the lima hardware needs:
> > > > > > > - a call to dev_pm_opp_set_clkname() during initialization because there
> > > > > > >       are two clocks on Mali-4x0 IPs. "core" is the one that actually clocks
> > > > > > >       the GPU so we need to control it using devfreq.
> > > > > > > - locking when reading or writing the devfreq statistics because (unlike
> > > > > > >       than panfrost) we have multiple PP and GP IRQs which may finish jobs
> > > > > > >       concurrently.
> > > > > >
> > > > > > I gave this a quick try on my RK3328, and the clock scaling indeed kicks
> > > > > > in nicely on the glmark2 scenes that struggle, however something appears
> > > > > > to be missing in terms of regulator association, as the appropriate OPP
> > > > > > voltages aren't reflected in the GPU supply (fortunately the initial
> > > > > > voltage seems close enough to that of the highest OPP not to cause major
> > > > > > problems, on my box at least). With panfrost on RK3399 I do see the
> > > > > > supply voltage scaling accordingly, but I don't know my way around
> > > > > > devfreq well enough to know what matters in the difference :/
> > > > > first of all: thank you for trying this out! :-)
> > > > >
> > > > > does your kernel include commit 221bc77914cbcc ("drm/panfrost: Use
> > > > > generic code for devfreq") for your panfrost test?
> > > > > if I understand the devfreq API correct then I suspect with that
> > > > > commit panfrost also won't change the voltage anymore.
> > > >
> > > > Oh, you're quite right - I was already considering that change as
> > > > ancient history, but indeed it's only in 5.5-rc, while that board is
> > > > still on 5.4.y release kernels. No wonder I couldn't make sense of how
> > > > the (current) code could possibly be working :)
> > > >
> > > > I'll try the latest -rc kernel tomorrow to confirm (now that PCIe is
> > > > hopefully fixed), but I'm already fairly confident you've called it
> > > > correctly.
> > > I just tested it with the lima driver (by undervolting the GPU by
> > > 0.05V) and it seems that dev_pm_opp_set_regulators is really needed.
> > > I'll fix this in the next version of this patch and also submit a fix
> > > for panfrost (I won't be able to test that though, so help is
> > > appreciated in terms of testing :))
> >
> > Yeah, I started hacking something up for panfrost yesterday, but at the
> > point of realising the core OPP code wants refactoring to actually
> > handle optional regulators without spewing errors, decided that was
> > crossing the line into "work" and thus could wait until next week :D
> I'm not sure what you mean, dev_pm_opp_set_regulators uses
> regulator_get_optional.
> doesn't that mean that it is optional already?

Indeed it does call regulator_get_optional(), but it then goes on to 
treat the absence of a supposedly-optional regulator as a hard failure. 
It doesn't seem very useful having a nice abstracted interface if users 
still end up have to dance around and duplicate half the parsing in 
order to work out whether it's worth calling or not - far better IMO if 
it could just successfully set/put zero regulators in the cases where 
the OPPs are behind a firmware/mailbox DVFS interface rather than 
explicit in-kernel clock/regulator control.

That said, given that I think the current lima/panfrost users should all 
be relatively simple with either 0 or 1 regulator, you could probably 
just special-case -ENODEV and accept a spurious error message sometimes 
for the sake of an immediate fix, then we can make general improvements 
to the interface separately afterwards.

Robin.