Re: [PATCH v2 4/7] drm/panfrost: Add support for a second regulator for the GPU

[Nicolas Boichat]

On Fri, Jan 10, 2020 at 7:30 PM Steven Price <steven.price@xxxxxxx> wrote:
>
> On 09/01/2020 19:49, Mark Brown wrote:
> > On Thu, Jan 09, 2020 at 04:53:02PM +0000, Steven Price wrote:
> >> On 09/01/2020 16:28, Mark Brown wrote:
> >>> On Thu, Jan 09, 2020 at 02:14:42PM +0000, Steven Price wrote:
> >
> >>>> I'm not sure if it's better, but could we just encode the list of
> >>>> regulators into device tree. I'm a bit worried about special casing an
> >>>> "sram" regulator given that other platforms might have a similar
> >>>> situation but call the second regulator a different name.
> >
> >>> Obviously the list of regulators bound on a given platform is encoded in
> >>> the device tree but you shouldn't really be relying on that to figure
> >>> out what to request in the driver - the driver should know what it's
> >>> expecting.
> >
> >> From a driver perspective we don't expect to have to worry about power
> >> domains/multiple regulators - the hardware provides a bunch of power
> >> registers to turn on/off various parts of the hardware and this should be
> >> linked (in hardware) to a PDC which sorts it out. The GPU/PDC handles the
> >> required sequencing. So it *should* be a case of turn power/clocks on and
> >> go.
> >
> > Ah, the well abstracted and consistent hardware with which we are all so
> > fortunate to work :) .  More seriously perhaps the thing to do here is
> > create a driver that provides a soft PDC and then push all the special
> > case handling into that?  It can then get instantiated based on the
> > compatible or perhaps represented directly in the device tree if that
> > makes sense.
>
> That makes sense to me.
>
> >> However certain integrations may have quirks such that there are physically
> >> multiple supplies. These are expected to all be turned on before using the
> >> GPU. Quite how this is best represented is something I'm not sure about.
> >
> > If they're always on and don't ever change then that's really easy to
> > represent in the DT without involving drivers, it's when you need to
> > actively manage them that it's more effort.
>
> Sorry, I should have been more clear. They are managed as a group - so
> either the entire GPU is powered off, or powered on. There's no support
> in Panfrost or mali_kbase for attempting to power part of the GPU.
>
> >>> Bear in mind that getting regulator stuff wrong can result
> >>> in physical damage to the system so it pays to be careful and to
> >>> consider that platform integrators have a tendency to rely on things
> >>> that just happen to work but aren't a good idea or accurate
> >>> representations of the system.  It's certainly *possible* to do
> >>> something like that, the information is there, but I would not in any
> >>> way recommend doing things that way as it's likely to not be robust.
> >
> >>> The possibility that the regulator setup may vary on other platforms
> >>> (which I'd expect TBH) does suggest that just requesting a bunch of
> >>> supply names optionally and hoping that we got all the ones that are
> >>> important on a given platform is going to lead to trouble down the line.
> >
> >> Certainly if we miss a regulator the GPU isn't going to work properly (some
> >> cores won't be able to power up successfully). However at the moment the
> >> driver will happily do this if someone provides it with a DT which includes
> >> regulators that it doesn't know about. So I'm not sure how adding special
> >> case code for a SoC would help here.
> >
> > I thought this SoC neeed to vary the voltage on both rails as part of
> > the power management?  Things like that can lead to hardware damage if
> > we go out of spec far enough for long enough - there can be requirements
> > like keeping one rail a certain voltage above another or whatever.
>
> Yes, you are correct. My concern is that a DT which specifies a new
> regulator (e.g. "sram2") would be accepted by an old kernel (because it
> wouldn't know to look for the new regulator) but wouldn't know to
> control the regulator. It could then create a situation which puts the
> board out of spec - potentially in a damaging way. Hence I'd like to
> express the regulator structure in such a way that old kernels wouldn't
> "half-work". Your "soft-PDC" approach would seem to fit that requirement.

FYI, I sent a v3 here: https://patchwork.kernel.org/patch/11331373/
that addresses _some_ of these concerns.

I'm not quite sure how to describe the regulators in a way that we can
check that the device tree does not specific extra ones (apart from
doing some string matching on all properties?), and I don't think I'm
best placed to implement the soft-PDC idea. See my comment on that
patch.

Thanks!