Re: [PATCH v2 5/7] drm/panfrost: Add support for multiple power domain support

[Ulf Hansson]

On Fri, 10 Jan 2020 at 02:53, Nicolas Boichat <drinkcat@xxxxxxxxxxxx> wrote:
>
> +Ulf to keep me honest on the power domains
>
> On Thu, Jan 9, 2020 at 10:08 PM Steven Price <steven.price@xxxxxxx> wrote:
> >
> > On 08/01/2020 05:23, Nicolas Boichat wrote:
> > > When there is a single power domain per device, the core will
> > > ensure the power domains are all switched on.
> > >
> > > However, when there are multiple ones, as in MT8183 Bifrost GPU,
> > > we need to handle them in driver code.
> > >
> > >
> > > Signed-off-by: Nicolas Boichat <drinkcat@xxxxxxxxxxxx>
> > > ---
> > >
> > > The downstream driver we use on chromeos-4.19 currently uses 2
> > > additional devices in device tree to accomodate for this [1], but
> > > I believe this solution is cleaner.
> >
> > I'm not sure what is best, but it seems odd to encode this into the Panfrost driver itself - it doesn't have any knowledge of what to do with these power domains. The naming of the domains looks suspiciously like someone thought that e.g. only half of the cores could be powered, but it doesn't look like that was implemented in the chromeos driver linked and anyway that is *meant* to be automatic in the hardware! (I.e. if you only power up one cores in one core stack then the PDC should only enable the power domain for that set of cores).
>
> This is actually implemented in the Chrome OS driver [1]. IMHO power
> domains are a bit confusing [2]:
>  i. If there's only 1 power domain in the device, then the core takes
> care of power on the domain (based on pm_runtime)
>  ii. If there's more than 1 power domain, then the device needs to
> link the domains manually.
>
> So the Chrome OS [1] driver takes approach (i), by creating 3 devices,
> each with 1 power domain that is switched on/off automatically using
> pm_runtime.
>
> This patch takes approach (ii) with device links to handle the extra domains.
>
> I believe the latter is more upstream-friendly, but, as always,
> suggestions welcome.

Apologies for the late reply. A few comments below.

If the device is partitioned across multiple PM domains (it may need
several power rails), then that should be described with the "multi PM
domain" approach in the DTS. As in (ii).

Using "device links" is however optional, as it may depend on the use
case. If all multiple PM domains needs to be powered on/off together,
then it's certainly recommended to use device links.

However, if the PM domains can be powered on/off independently (one
can be on while another is off), then it's probably easier to operate
directly with runtime PM, on the returned struct *device from
dev_pm_domain_attach_by_id().

Also note, there is dev_pm_domain_attach_by_name(), which allows us to
specify a name for the PM domain in the DTS, rather than using an
index. This may be more future proof to use.

[...]

Hope this helps.

Kind regards
Uffe