linux-kernel - Re: [PATCH 2/9] soc: samsung: exynos-pmu: Add exynos_pmu

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Message-ID: <CADrjBPoYpVDYptJSEd+=5x+eRnsuZzm4uOUokNEpEX2dxMGtiQ@mail.gmail.com>
Date: Thu, 25 Jan 2024 10:48:35 +0000
From: Peter Griffin <peter.griffin@...aro.org>
To: Sam Protsenko <semen.protsenko@...aro.org>
Cc: arnd@...db.de, robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org, 
	linux@...ck-us.net, wim@...ux-watchdog.org, conor+dt@...nel.org, 
	alim.akhtar@...sung.com, jaewon02.kim@...sung.com, chanho61.park@...sung.com, 
	kernel-team@...roid.com, tudor.ambarus@...aro.org, andre.draszik@...aro.org, 
	saravanak@...gle.com, willmcvicker@...gle.com, linux-fsd@...la.com, 
	linux-watchdog@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-samsung-soc@...r.kernel.org
Subject: Re: [PATCH 2/9] soc: samsung: exynos-pmu: Add exynos_pmu_update/read/write
 APIs and SoC quirks

Hi Sam,

Thanks for your review feedback!

On Wed, 24 Jan 2024 at 20:23, Sam Protsenko <semen.protsenko@...aro.org> wrote:
>
> On Wed, Jan 24, 2024 at 4:02 AM Peter Griffin <peter.griffin@...aro.org> wrote:
> >
> > Hi Sam,
> >
> > Thanks for the review feedback.
> >
> > On Tue, 23 Jan 2024 at 18:56, Sam Protsenko <semen.protsenko@...aro.org> wrote:
> > >
> > > On Mon, Jan 22, 2024 at 4:57 PM Peter Griffin <peter.griffin@...aro.org> wrote:
> > > >
> > > > Newer Exynos SoCs have atomic set/clear bit hardware for PMU registers as
> > > > these registers can be accessed by multiple masters. Some platforms also
> > > > protect the PMU registers for security hardening reasons so they can't be
> > > > written by normal world and are only write acessible in el3 via a SMC call.
> > > >
> > > > Add support for both of these usecases using SoC specific quirks that are
> > > > determined from the DT compatible string.
> > > >
> > > > Drivers which need to read and write PMU registers should now use these
> > > > new exynos_pmu_*() APIs instead of obtaining a regmap using
> > > > syscon_regmap_lookup_by_phandle()
> > > >
> > > > Depending on the SoC specific quirks, the exynos_pmu_*() APIs will access
> > > > the PMU register in the appropriate way.
> > > >
> > > > Signed-off-by: Peter Griffin <peter.griffin@...aro.org>
> > > > ---
> > > >  drivers/soc/samsung/exynos-pmu.c       | 209 ++++++++++++++++++++++++-
> > > >  drivers/soc/samsung/exynos-pmu.h       |   4 +
> > > >  include/linux/soc/samsung/exynos-pmu.h |  28 ++++
> > > >  3 files changed, 234 insertions(+), 7 deletions(-)
> > > >
> > >
> > > [snip]
> > >
> > > > +
> > > > +int exynos_pmu_update_bits(unsigned int offset, unsigned int mask,
> > > > +                          unsigned int val)
> > > > +{
> > > > +       if (pmu_context->pmu_data &&
> > > > +           pmu_context->pmu_data->quirks & QUIRK_PMU_ALIVE_WRITE_SEC)
> > > > +               return rmw_priv_reg(pmu_context->pmu_base_pa + offset,
> > > > +                                   mask, val);
> > > > +
> > > > +       return regmap_update_bits(pmu_context->pmureg, offset, mask, val);
> > > > +}
> > > > +EXPORT_SYMBOL(exynos_pmu_update_bits);
> > > > +
> > >
> > > This seems a bit hacky, from the design perspective. This way the user
> > > will have to worry about things like driver dependencies, making sure
> > > everything is instantiated in a correct order, etc. It also hides the
> > > details otherwise visible through "syscon-phandle" property in the
> > > device tree.
> >
> > In v2 I will keep the phandle to pmu_system_controller in DT, and add
> > some -EPROBE_DEFER logic (See my email with Krzysztof).
> >
> > > Can we instead rework it by overriding regmap
> > > implementation for Exynos specifics, and then continue to use it in
> > > the leaf drivers via "syscon-phandle" property?
> >
> > I did look at that possibility first, as like you say it would avoid
> > updating the leaf drivers to use the new API. Unfortunately a SMC
> > backend to regmap was already tried and nacked upstream pretty hard.
> > See here https://lore.kernel.org/lkml/20210723163759.GI5221@sirena.org.uk/T/
> >
>
> Oh, I didn't mean creating a new regmap implementation :) To
> illustrate what I meant, please look at these:
>
>   - drivers/mfd/altera-sysmgr.c
>   - altr_sysmgr_regmap_lookup_by_phandle()
>   - arch/arm64/boot/dts/altera/socfpga_stratix10.dtsi
>   - drivers/mmc/host/dw_mmc-pltfm.c

Thanks for the pointers :) I hadn't spotted this when looking
previously. I did find the previous threads I linked to and (it
appears wrongly concluded) that such a regmap SMC would not be
acceptable.
>
> They basically implement their own regmap operations (with smcc too)
> in their syscon implementation. So they can actually reference that
> syscon as phandle in device tree and avoid exporting and calling
> read/write operations (which I think looks hacky). Instead they use
> altr_sysmgr_regmap_lookup_by_phandle() to get their regmap (which
> performs smcc), and then they just use regular regmap_read() /
> regmap_write or whatever functions to operate on their regmap object.
> That's what I meant by "overriding" the regmap.
>
> Do you think this approach would be clearer and more "productizable"
> so to speak? Just a thought.

Keeping it as a regmap was certainly always my preference. I'll try
and re-work it in a similar way and see if I hit any blocking issues.

Thanks,

Peter.