Re: [PATCH v3 0/5] Reset driver for IMX8MQ VPU hardware block

Philipp Zabel
2021-03-03T19:09:31Z

On Wed, 2021-03-03 at 16:20 +0100, Benjamin Gaignard wrote:
> Le 03/03/2021 à 15:17, Philipp Zabel a écrit :
> > Hi Benjamin,
> >
> > On Mon, 2021-03-01 at 16:17 +0100, Benjamin Gaignard wrote:
> > > The two VPUs inside IMX8MQ share the same control block which can be see
> > > as a reset hardware block.
> > This isn't a reset controller though. The control block also contains
> > clock gates of some sort and a filter register for the featureset fuses.
> > Those shouldn't be manipulated via the reset API.
>
> They are all part of the control block and of the reset process for this
> hardware that why I put them here. I guess it is border line :-)

I'm pushing back to keep the reset control framework focused on
controlling reset lines. Every side effect (such as the asymmetric clock
ungating) in a random driver makes it harder to reason about behaviour
at the API level, and to review patches for hardware I am not familiar
with.

> > > In order to be able to add the second VPU (for HECV decoding) it will be
> > > more handy if the both VPU drivers instance don't have to share the
> > > control block registers. This lead to implement it as an independ reset
> > > driver and to change the VPU driver to use it.
> > Why not switch to a syscon regmap for the control block? That should
> > also allow to keep backwards compatibility with the old binding with
> > minimal effort.
>
> I will give a try in this direction.

Thank you.

> > > Please note that this series break the compatibility between the DTB and
> > > kernel. This break is limited to IMX8MQ SoC and is done when the driver
> > > is still in staging directory.
> > I know in this case we are pretty sure there are no users of this
> > binding except for a staging driver, but it would still be nice to keep
> > support for the deprecated binding, to avoid the requirement of updating
> > kernel and DT in lock-step.
>
> If I want to use a syscon (or a reset) the driver must not ioremap the "ctrl"
> registers. It means that "ctrl" has to be removed from the driver requested
> reg-names (imx8mq_reg_names[]). Doing that break the kernel/DT compatibility.
> Somehow syscon and "ctrl" are exclusive.

The way the driver is set up currently, yes. You could add a bit of
platform specific probe code, though, that would set up the regmap
either by calling
syscon_regmap_lookup_by_phandle();
for the new binding, or, if the phandle is not available, fall back to
platform_get_resource_byname(..., "ctrl");
devm_ioremap_resource();
devm_regmap_init_mmio();
for the old binding.
The actual codec .reset and variant .runtime_resume ops could be
identical then.

regards
Philipp