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
