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: <CADrjBPrWH8uFrFmn_CZpr+fAnPrzbDT4i9XuMXJqKfzeouPpKg@mail.gmail.com>
Date: Wed, 7 Feb 2024 11:42:03 +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,

Thanks for your review feedback.

On Mon, 5 Feb 2024 at 13:13, Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> On 01/02/2024 13:51, Peter Griffin wrote:
> > 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.
>
> Hm, I don't quite get why you cannot use of_find_device_by_node()?

of_find_device_by_node() returns a platform_device, even if the driver
hasn't probed. Whereas driver_find_device_by_of_node() iterates
devices bound to a driver.

If using of_find_device_by_node() API I could check the result of
platform_get_drvdata(), and -EPROBE_DEFER if NULL (that pattern seems
to be used by a few drivers). But that AFAIK only guarantees you
reached the platform_set_drvdata() call in your driver probe()
function, not that it has completed.

IMHO the drivers using driver_find_device_by_of_node() for probe
deferral are doing it more robustly than those using
of_find_device_by_node()  and checking if platform_get_drvdata() is
NULL.

Or is there some other way you had in mind of using
of_find_device_by_node() I've not thought of to implement this?

Thanks,

Peter.