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: <99828589-c0b5-456d-b250-6ad3e6085a91@linaro.org>
Date: Mon, 5 Feb 2024 14:13:13 +0100
From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To: Peter Griffin <peter.griffin@...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

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()?

Best regards,
Krzysztof