Re: [PATCH v2 5/5] firmware: QCOM_SCM: Allow qcom_scm driver to be loadable as a permenent module

[Will Deacon]

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@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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/

Will