linux-kernel - Re: [PATCH v2 1/2] soc: samsung: exynos-pmu: Add regmap support for SoCs that protect PMU regs

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Message-ID: <CADrjBPoQmTRsFYRtxBxdvAoKK816O8XN3=hOJ3vBt8wbbbk-=Q@mail.gmail.com>
Date: Thu, 1 Feb 2024 12:51:47 +0000
From: Peter Griffin <peter.griffin@...aro.org>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc: arnd@...db.de, linux@...ck-us.net, wim@...ux-watchdog.org, 
	alim.akhtar@...sung.com, jaewon02.kim@...sung.com, semen.protsenko@...aro.org, 
	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 v2 1/2] soc: samsung: exynos-pmu: Add regmap support for
 SoCs that protect PMU regs

Hi Krzysztof,

On Tue, 30 Jan 2024 at 16:01, Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> On 29/01/2024 22:19, Peter Griffin wrote:
> > Some Exynos based SoCs like Tensor gs101 protect the PMU registers for
> > security hardening reasons so that they are only accessible in el3 via an
> > SMC call.
> >
> > As most Exynos drivers that need to write PMU registers currently obtain a
> > regmap via syscon (phys, pinctrl, watchdog). Support for the above usecase
> > is implemented in this driver using a custom regmap similar to syscon to
> > handle the SMC call. Platforms that don't secure PMU registers, get a mmio
> > regmap like before. As regmaps abstract out the underlying register access
> > changes to the leaf drivers are minimal.
> >
> > A new API exynos_get_pmu_regmap_by_phandle() is provided for leaf drivers
> > that currently use syscon_regmap_lookup_by_phandle(). This also handles
> > deferred probing.
> >
> > Signed-off-by: Peter Griffin <peter.griffin@...aro.org>
> > ---
> >  drivers/soc/samsung/exynos-pmu.c       | 227 ++++++++++++++++++++++++-
> >  include/linux/soc/samsung/exynos-pmu.h |  10 ++
> >  2 files changed, 236 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/soc/samsung/exynos-pmu.c b/drivers/soc/samsung/exynos-pmu.c
> > index 250537d7cfd6..7bcc144e53a2 100644
> > --- a/drivers/soc/samsung/exynos-pmu.c
> > +++ b/drivers/soc/samsung/exynos-pmu.c
> > @@ -5,6 +5,7 @@
> >  //
> >  // Exynos - CPU PMU(Power Management Unit) support
> >
> > +#include <linux/arm-smccc.h>
> >  #include <linux/of.h>
> >  #include <linux/of_address.h>
> >  #include <linux/mfd/core.h>
> > @@ -12,20 +13,159 @@
> >  #include <linux/of_platform.h>
> >  #include <linux/platform_device.h>
> >  #include <linux/delay.h>
> > +#include <linux/regmap.h>
> >
> >  #include <linux/soc/samsung/exynos-regs-pmu.h>
> >  #include <linux/soc/samsung/exynos-pmu.h>
> >
> >  #include "exynos-pmu.h"
> >
> > +static struct platform_driver exynos_pmu_driver;
>
> I don't understand why do you need it. You can have only one
> pmu_context. The moment you probe second one, previous becomes invalid.
>
> I guess you want to parse phandle and check if just in case if it points
> to the right device, but still the original code is not ready for two
> PMU devices. I say either this problem should be solved entirely,
> allowing two devices, or just compare device node from phandle with
> device node of exynos_pmu_context->dev and return -EINVAL on mismatches.

Apologies I didn't answer your original question. This wasn't about
having partial support for multiple pmu devices. It is being used by
driver_find_device_by_of_node() in exynos_get_pmu_regmap_by_phandle()
to determine that the exynos-pmu device has probed and therefore a
pmu_context exists and a regmap has been created and can be returned
to the caller (as opposed to doing a -EPROBE_DEFER).

Is there some better/other API you recommend for this purpose? Just
checking pmu_context directly seems racy, so I don't think we should
do that.

Peter

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