Date:   Wed, 28 Oct 2020 14:51:03 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Will Deacon <will@...nel.org>, John Stultz <john.stultz@...aro.org>
Cc:     Maulik Shah <mkshah@...eaurora.org>,
        Jason Cooper <jason@...edaemon.net>,
        Saravana Kannan <saravanak@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Lina Iyer <ilina@...eaurora.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        iommu@...ts.linux-foundation.org, Andy Gross <agross@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH v2 5/5] firmware: QCOM_SCM: Allow qcom_scm driver to be
 loadable as a permenent module

On 2020-10-28 13:51, Will Deacon wrote:
> On Tue, Oct 27, 2020 at 10:53:47PM -0700, John Stultz wrote:
>> On Mon, Jul 13, 2020 at 1:41 PM Will Deacon <will@...nel.org> wrote:
>>> On Fri, Jul 10, 2020 at 03:21:53PM -0700, John Stultz wrote:
>>>> On Fri, Jul 10, 2020 at 12:54 AM Will Deacon <will@...nel.org> wrote:
>>>>> On Thu, Jul 09, 2020 at 08:28:45PM -0700, John Stultz wrote:
>>>>>> On Thu, Jul 2, 2020 at 7:18 AM Will Deacon <will@...nel.org> wrote:
>>>>>>> On Thu, Jun 25, 2020 at 12:10:39AM +0000, John Stultz wrote:
>>>>>>>> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
>>>>>>>> index b510f67dfa49..714893535dd2 100644
>>>>>>>> --- a/drivers/iommu/Kconfig
>>>>>>>> +++ b/drivers/iommu/Kconfig
>>>>>>>> @@ -381,6 +381,7 @@ config SPAPR_TCE_IOMMU
>>>>>>>>   config ARM_SMMU
>>>>>>>>        tristate "ARM Ltd. System MMU (SMMU) Support"
>>>>>>>>        depends on (ARM64 || ARM || (COMPILE_TEST && !GENERIC_ATOMIC64)) && MMU
>>>>>>>> +     depends on QCOM_SCM || !QCOM_SCM #if QCOM_SCM=m this can't be =y
>>>>>>>>        select IOMMU_API
>>>>>>>>        select IOMMU_IO_PGTABLE_LPAE
>>>>>>>>        select ARM_DMA_USE_IOMMU if ARM
>>>>>>>
>>>>>>> This looks like a giant hack. Is there another way to handle this?
>>>>>>
>>>>>> Sorry for the slow response here.
>>>>>>
>>>>>> So, I agree the syntax looks strange (requiring a comment obviously
>>>>>> isn't a good sign), but it's a fairly common way to ensure drivers
>>>>>> don't get built in if they optionally depend on another driver that
>>>>>> can be built as a module.
>>>>>>    See "RFKILL || !RFKILL", "EXTCON || !EXTCON", or "USB_GADGET ||
>>>>>> !USB_GADGET" in various Kconfig files.
>>>>>>
>>>>>> I'm open to using a different method, and in a different thread you
>>>>>> suggested using something like symbol_get(). I need to look into it
>>>>>> more, but that approach looks even more messy and prone to runtime
>>>>>> failures. Blocking the unwanted case at build time seems a bit cleaner
>>>>>> to me, even if the syntax is odd.
>>>>>
>>>>> Maybe just split it out then, so that the ARM_SMMU entry doesn't have this,
>>>>> as that driver _really_ doesn't care about SoC details like this. In other
>>>>> words, add a new entry along the lines of:
>>>>>
>>>>>          config ARM_SMMU_QCOM_IMPL
>>>>>          default y
>>>>>          #if QCOM_SCM=m this can't be =y
>>>>>          depends on ARM_SMMU & (QCOM_SCM || !QCOM_SCM)
>>>>>
>>>>> and then have arm-smmu.h provide a static inline qcom_smmu_impl_init()
>>>>> which returns -ENODEV if CONFIG_ARM_SMMU_QCOM_IMPL=n and hack the Makefile
>>>>> so that we don't bother to compile arm-smmu-qcom.o in that case.
>>>>>
>>>>> Would that work?
>>>>
>>>> I think this proposal still has problems with the directionality of the call.
>>>>
>>>> The arm-smmu-impl.o calls to arm-smmu-qcom.o which calls qcom_scm.o
>>>> So if qcom_scm.o is part of a module, the calling code in
>>>> arm-smmu-qcom.o also needs to be a module, which means CONFIG_ARM_SMMU
>>>> needs to be a module.
>>>>
>>>> I know you said the arm-smmu driver doesn't care about SoC details,
>>>> but the trouble is that currently the arm-smmu driver does directly
>>>> call the qcom-scm code. So it is a real dependency. However, if
>>>> QCOM_SCM is not configured, it calls stubs and that's ok.  In that
>>>> way, the "depends on QCOM_SCM || !QCOM_SCM" line actually makes sense.
>>>> It looks terrible because we're used to boolean logic, but it's
>>>> ternary.
>>>
>>> Yes, it looks ugly, but the part I really have issues with is that building
>>> QCOM_SCM=m and ARM_SMMU=y is perfectly fine if you don't run on an SoC
>>> with the qcom implementation. I don't see why we need to enforce things
>>> here beyond making sure that all selectable permutations _build_ and
>>> fail gracefully at runtime on the qcom SoC if SCM isn't available.
>>
>> Hey Will,
>>    Sorry to dredge up this old thread. I've been off busy with other
>> things and didn't get around to trying to rework this until now.
>>
>> Unfortunately I'm still having some trouble coming up with a better
>> solution. Initially I figured I'd rework the qcom_scm driver to, so
>> that we have the various qcom_scm_* as inline functions, which call
>> out to function pointers that the qcom_scm driver would register when
>> the module loaded (Oof, and unfortunately there are a *ton* qcom_scm_*
>> functions so its a bunch of churn).
>>
>> The trouble I realized with that approach is that if the ARM_SMMU code
>> is built in, then it may try to use the qcom_scm code before the
>> module loads and sets those function pointers. So while it would build
>> ok, the issue would be when the arm_smmu_device_reset() is done by
>> done on arm_smmu_device_probe(), it wouldn't actually call the right
>> code.  There isn't a really good way to deal with the module loading
>> at some random time after arm_smmu_device_probe() completes.
>>
>> This is the benefit of the module symbol dependency tracking: If the
>> arm_smmu.ko calls symbols in qcom_scm.ko then qcom_scm.ko has to load
>> first.
>> But if arm_smmu is built in, I haven't found a clear mechanism to
>> force qcom_scm to load before we probe, if it's configured as a
>> module.
>>
>> I also looked into the idea of reworking the arm-smmu-impl code to be
>> modular instead, and while it does provide a similar method of using
>> function pointers to minimize the amount of symbols the arm-smmu code
>> needs to know about, the initialization call path is
>> arm_smmu_device_probe -> arm_smmu_impl_init -> qcom_smmu_impl_init. So
>> it doesn't really allow for dynamic registration of implementation
>> modules at runtime.
>>
>> So I'm sort of stewing on maybe trying to rework the directionality,
>> so the arm-smmu-qcom.o code probes and calls arm_smmu_impl_init and
>> that is what initializes the arm_smmu_device_probe logic?
>>
>> Alternatively, I'm considering trying to switch the module dependency
>> annotation so that the CONFIG_QCOM_SCM modularity depends on ARM_SMMU
>> being a module. But that is sort of putting the restriction on the
>> callee instead of the caller (sort of flipping the meaning of the
>> depends), which feels prone to later trouble (and with multiple users
>> of CONFIG_QCOM_SCM needing similar treatment, it would make it
>> difficult to discover the right combination of configs needed to allow
>> it to be a module).
>>
>> Anyway, I wanted to reach out to see if you had any further ideas
>> here. Sorry for letting such a large time gap pass!
> 
> Well we can always go with your original hack, if it helps?
> 
> https://lore.kernel.org/linux-iommu/20200714075603.GE4277@willie-the-truck/

Hmm, perhaps I'm missing something here, but even if the config options 
*do* line up, what prevents arm-smmu probing before qcom-scm and 
dereferencing NULL in qcom_scm_qsmmu500_wait_safe_toggle() before __scm 
is initialised?

Robin.

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