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

[Adam Ford <aford173@gmail.com>]

On Wed, Mar 3, 2021 at 5:24 PM Philipp Zabel <p.zabel@pengutronix.de> wrote:
>
> 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.

This driver is very similar to several other patches for clk_blk
control [1] which contain both resets and clock-enables on the
i.MX8MP, i.MX8MM and i.MX8MN.  In those cases, there are some specific
power domain controls that are needed, but I wonder if the approach to
creating resets and clock enables could be used in a similar way if
the IMX8MQ doesn't have the same quirks.  In the case of the i.MX8M
Mini, I think it has the same VPU.

[1] - https://patchwork.kernel.org/project/linux-clk/patch/1599560691-3763-12-git-send-email-abel.vesa@nxp.com/

adam
> >
> > 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
>
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