Messages in this thread | | | Subject | Re: [PATCH v2 5/5] firmware: QCOM_SCM: Allow qcom_scm driver to be loadable as a permenent module | From | Robin Murphy <> | Date | Wed, 28 Oct 2020 14:51:03 +0000 |
| |
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@kernel.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@kernel.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@kernel.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.
| |