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

[Martin Blumenstingl]

Hi Robin,

On Wed, Jan 1, 2020 at 1:55 PM Robin Murphy <robin.murphy at arm.com> wrote:
>
> 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 at arm.com> 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 at arm.com> 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 at arm.com> 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.
thank you for the explanation
I'll experience this case on newer Amlogic SoCs where regulators are
hidden behind SCPI firmware. so far I have only tested the case
without OPP-table on those SoCs.

> 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.
OK, I'll be working on this next week again
if you get to fix panfrost earlier then please Cc me so we don't duplicate work


Martin