[PATCH] iommu/arm: Allow disabling Qualcomm support in arm_smmu_v3

[PATCH] iommu/arm: Allow disabling Qualcomm support in arm_smmu_v3

[Aaron Kling via B4 Relay]

From: Aaron Kling <webgeek1234@gmail.com>

If ARCH_QCOM is enabled when building arm_smmu_v3, a dependency on
qcom-scm is added, which currently cannot be disabled. Add a prompt to
ARM_SMMU_QCOM to allow disabling this dependency.

Fixes: 0f0f80d9d5db ("iommu/arm: fix ARM_SMMU_QCOM compilation")
Cc: stable@vger.kernel.org
Signed-off-by: Aaron Kling <webgeek1234@gmail.com>
---
 drivers/iommu/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index ec1b5e32b9725bc1104d10e5d7a32af7b211b50a..cca0825551959e3f37cc2ea41aeae526fdb73312 100644
--- a/drivers/iommu/Kconfig
+++ b/drivers/iommu/Kconfig
@@ -381,6 +381,7 @@ config ARM_SMMU_MMU_500_CPRE_ERRATA
 
 config ARM_SMMU_QCOM
 	def_tristate y
+	prompt "Qualcomm SMMUv3 Support"
 	depends on ARM_SMMU && ARCH_QCOM
 	select QCOM_SCM
 	help

---
base-commit: 1110ce6a1e34fe1fdc1bfe4ad52405f327d5083b
change-id: 20250310-b4-qcom-smmu-d4ccaf66a1ce

Best regards,
-- 
Aaron Kling <webgeek1234@gmail.com>

Re: [PATCH] iommu/arm: Allow disabling Qualcomm support in arm_smmu_v3

[Robin Murphy]

On 2025-03-10 6:11 am, Aaron Kling via B4 Relay wrote:
> From: Aaron Kling <webgeek1234@gmail.com>
> 
> If ARCH_QCOM is enabled when building arm_smmu_v3,

This has nothing to do with SMMUv3, though?

> a dependency on
> qcom-scm is added, which currently cannot be disabled. Add a prompt to
> ARM_SMMU_QCOM to allow disabling this dependency.

Why is that an issue - what problem arises from having the SCM driver 
enabled? AFAICS it's also selected by plenty of other drivers including 
pretty fundamental ones like pinctrl. If it is somehow important to 
exclude the SCM driver, then I can't really imagine what the use-case 
would be for building a kernel which won't work on most Qualcomm 
platforms but not simply disabling ARCH_QCOM...

Thanks,
Robin.

> Fixes: 0f0f80d9d5db ("iommu/arm: fix ARM_SMMU_QCOM compilation")
> Cc: stable@vger.kernel.org
> Signed-off-by: Aaron Kling <webgeek1234@gmail.com>
> ---
>   drivers/iommu/Kconfig | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> index ec1b5e32b9725bc1104d10e5d7a32af7b211b50a..cca0825551959e3f37cc2ea41aeae526fdb73312 100644
> --- a/drivers/iommu/Kconfig
> +++ b/drivers/iommu/Kconfig
> @@ -381,6 +381,7 @@ config ARM_SMMU_MMU_500_CPRE_ERRATA
>   
>   config ARM_SMMU_QCOM
>   	def_tristate y
> +	prompt "Qualcomm SMMUv3 Support"
>   	depends on ARM_SMMU && ARCH_QCOM
>   	select QCOM_SCM
>   	help
> 
> ---
> base-commit: 1110ce6a1e34fe1fdc1bfe4ad52405f327d5083b
> change-id: 20250310-b4-qcom-smmu-d4ccaf66a1ce
> 
> Best regards,

Re: [PATCH] iommu/arm: Allow disabling Qualcomm support in arm_smmu_v3

[Aaron Kling]

On Mon, Mar 10, 2025 at 7:42 AM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2025-03-10 6:11 am, Aaron Kling via B4 Relay wrote:
> > From: Aaron Kling <webgeek1234@gmail.com>
> >
> > If ARCH_QCOM is enabled when building arm_smmu_v3,
>
> This has nothing to do with SMMUv3, though?
>
> > a dependency on
> > qcom-scm is added, which currently cannot be disabled. Add a prompt to
> > ARM_SMMU_QCOM to allow disabling this dependency.
>
> Why is that an issue - what problem arises from having the SCM driver
> enabled? AFAICS it's also selected by plenty of other drivers including
> pretty fundamental ones like pinctrl. If it is somehow important to
> exclude the SCM driver, then I can't really imagine what the use-case
> would be for building a kernel which won't work on most Qualcomm
> platforms but not simply disabling ARCH_QCOM...
>

I am working with the android kernel. The more recent setup enables a
minimal setup of configs in a core kernel that works across all
supported arch's, then requires further support to all be modules. I
specifically am working with tegra devices. But as ARCH_QCOM is
enabled in the core defconfig, when I build smmuv3 as a module, I end
up with a dependency on qcom-scm which gets built as an additional
module. And it would be preferable to not require qcom modules to boot
a tegra device.

Sincerely,
Aaron

> Thanks,
> Robin.
>
> > Fixes: 0f0f80d9d5db ("iommu/arm: fix ARM_SMMU_QCOM compilation")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Aaron Kling <webgeek1234@gmail.com>
> > ---
> >   drivers/iommu/Kconfig | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> > index ec1b5e32b9725bc1104d10e5d7a32af7b211b50a..cca0825551959e3f37cc2ea41aeae526fdb73312 100644
> > --- a/drivers/iommu/Kconfig
> > +++ b/drivers/iommu/Kconfig
> > @@ -381,6 +381,7 @@ config ARM_SMMU_MMU_500_CPRE_ERRATA
> >
> >   config ARM_SMMU_QCOM
> >       def_tristate y
> > +     prompt "Qualcomm SMMUv3 Support"
> >       depends on ARM_SMMU && ARCH_QCOM
> >       select QCOM_SCM
> >       help
> >
> > ---
> > base-commit: 1110ce6a1e34fe1fdc1bfe4ad52405f327d5083b
> > change-id: 20250310-b4-qcom-smmu-d4ccaf66a1ce
> >
> > Best regards,
>

Re: [PATCH] iommu/arm: Allow disabling Qualcomm support in arm_smmu_v3

[Robin Murphy]

On 2025-03-10 4:45 pm, Aaron Kling wrote:
> On Mon, Mar 10, 2025 at 7:42 AM Robin Murphy <robin.murphy@arm.com> wrote:
>>
>> On 2025-03-10 6:11 am, Aaron Kling via B4 Relay wrote:
>>> From: Aaron Kling <webgeek1234@gmail.com>
>>>
>>> If ARCH_QCOM is enabled when building arm_smmu_v3,
>>
>> This has nothing to do with SMMUv3, though?
>>
>>> a dependency on
>>> qcom-scm is added, which currently cannot be disabled. Add a prompt to
>>> ARM_SMMU_QCOM to allow disabling this dependency.
>>
>> Why is that an issue - what problem arises from having the SCM driver
>> enabled? AFAICS it's also selected by plenty of other drivers including
>> pretty fundamental ones like pinctrl. If it is somehow important to
>> exclude the SCM driver, then I can't really imagine what the use-case
>> would be for building a kernel which won't work on most Qualcomm
>> platforms but not simply disabling ARCH_QCOM...
>>
> 
> I am working with the android kernel. The more recent setup enables a
> minimal setup of configs in a core kernel that works across all
> supported arch's, then requires further support to all be modules. I
> specifically am working with tegra devices. But as ARCH_QCOM is
> enabled in the core defconfig, when I build smmuv3 as a module, I end
> up with a dependency on qcom-scm which gets built as an additional
> module. And it would be preferable to not require qcom modules to boot
> a tegra device.

That just proves my point though - if you disable ARM_SMMU_QCOM in that 
context then you've got a kernel which won't work properly on Qualcomm 
platforms, so you may as well have just disabled ARCH_QCOM anyway. In 
fact the latter is objectively better since it then would not break the 
fundamental premise of "a core kernel that works across all supported 
arch's" :/

Maybe if you can find a viable way to separate out all the arm-smmu-qcom 
stuff into its own sub-module which only loads when needed, or possibly 
make SCM a soft-dep (given that we already have to cope with it being 
loaded but not initialised yet) then that might be a reasonable change 
to make; as it stands, I don't think this patch is. And it's definitely 
not a stable "fix" either way.

But frankly, weird modules happen. Why the heck is parport_pc currently 
loaded on my AArch64 workstation!? I can't even begin to imagine, but 
I'll live...

Thanks,
Robin.

Re: [PATCH] iommu/arm: Allow disabling Qualcomm support in arm_smmu_v3

[Aaron Kling]

On Mon, Mar 10, 2025 at 2:52 PM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2025-03-10 4:45 pm, Aaron Kling wrote:
> > On Mon, Mar 10, 2025 at 7:42 AM Robin Murphy <robin.murphy@arm.com> wrote:
> >>
> >> On 2025-03-10 6:11 am, Aaron Kling via B4 Relay wrote:
> >>> From: Aaron Kling <webgeek1234@gmail.com>
> >>>
> >>> If ARCH_QCOM is enabled when building arm_smmu_v3,
> >>
> >> This has nothing to do with SMMUv3, though?
> >>
> >>> a dependency on
> >>> qcom-scm is added, which currently cannot be disabled. Add a prompt to
> >>> ARM_SMMU_QCOM to allow disabling this dependency.
> >>
> >> Why is that an issue - what problem arises from having the SCM driver
> >> enabled? AFAICS it's also selected by plenty of other drivers including
> >> pretty fundamental ones like pinctrl. If it is somehow important to
> >> exclude the SCM driver, then I can't really imagine what the use-case
> >> would be for building a kernel which won't work on most Qualcomm
> >> platforms but not simply disabling ARCH_QCOM...
> >>
> >
> > I am working with the android kernel. The more recent setup enables a
> > minimal setup of configs in a core kernel that works across all
> > supported arch's, then requires further support to all be modules. I
> > specifically am working with tegra devices. But as ARCH_QCOM is
> > enabled in the core defconfig, when I build smmuv3 as a module, I end
> > up with a dependency on qcom-scm which gets built as an additional
> > module. And it would be preferable to not require qcom modules to boot
> > a tegra device.
>
> That just proves my point though - if you disable ARM_SMMU_QCOM in that
> context then you've got a kernel which won't work properly on Qualcomm
> platforms, so you may as well have just disabled ARCH_QCOM anyway. In
> fact the latter is objectively better since it then would not break the
> fundamental premise of "a core kernel that works across all supported
> arch's" :/

I'm not sure this is entirely true. Google's GKI mandates a fixed core
kernel Image. This has the minimal configs that can't be built as
modules. Then each device build is supposed to build independent sets
of modules via defconfig snippets that support the rest of the
hardware. Then what gets booted on a device is a prebuilt core kernel
image provided by Google, plus the modules built by the vendor. In
this setup, qcom-scm and ARM_SMMU_QCOM are modules and not part of the
core kernel. For a qcom target, arm_smmu_v3 would be built with
ARM_SMMU_QCOM. But then any non-qcom target that needs arm_smmu_v3
currently builds and deps qcom-scm. But there's no technical reason
they need that dep.
>
> Maybe if you can find a viable way to separate out all the arm-smmu-qcom
> stuff into its own sub-module which only loads when needed, or possibly
> make SCM a soft-dep (given that we already have to cope with it being
> loaded but not initialised yet) then that might be a reasonable change
> to make; as it stands, I don't think this patch is. And it's definitely
> not a stable "fix" either way.

The cc stable could be dropped. But I'm working with android forks of
6.6 and 6.12 currently, so I was hoping to get this pushed back to
stable, which would eventually filter its way over there.

>
> But frankly, weird modules happen. Why the heck is parport_pc currently
> loaded on my AArch64 workstation!? I can't even begin to imagine, but
> I'll live...

This is fair. But I've got to try at least to make the module
spaghetti make sense. If no one tries, it just gets worse.
>
> Thanks,
> Robin.

Sincerely,
Aaron

Re: [PATCH] iommu/arm: Allow disabling Qualcomm support in arm_smmu_v3

[Robin Murphy]

On 10/03/2025 8:15 pm, Aaron Kling wrote:
> On Mon, Mar 10, 2025 at 2:52 PM Robin Murphy <robin.murphy@arm.com> wrote:
>>
>> On 2025-03-10 4:45 pm, Aaron Kling wrote:
>>> On Mon, Mar 10, 2025 at 7:42 AM Robin Murphy <robin.murphy@arm.com> wrote:
>>>>
>>>> On 2025-03-10 6:11 am, Aaron Kling via B4 Relay wrote:
>>>>> From: Aaron Kling <webgeek1234@gmail.com>
>>>>>
>>>>> If ARCH_QCOM is enabled when building arm_smmu_v3,
>>>>
>>>> This has nothing to do with SMMUv3, though?
>>>>
>>>>> a dependency on
>>>>> qcom-scm is added, which currently cannot be disabled. Add a prompt to
>>>>> ARM_SMMU_QCOM to allow disabling this dependency.
>>>>
>>>> Why is that an issue - what problem arises from having the SCM driver
>>>> enabled? AFAICS it's also selected by plenty of other drivers including
>>>> pretty fundamental ones like pinctrl. If it is somehow important to
>>>> exclude the SCM driver, then I can't really imagine what the use-case
>>>> would be for building a kernel which won't work on most Qualcomm
>>>> platforms but not simply disabling ARCH_QCOM...
>>>>
>>>
>>> I am working with the android kernel. The more recent setup enables a
>>> minimal setup of configs in a core kernel that works across all
>>> supported arch's, then requires further support to all be modules. I
>>> specifically am working with tegra devices. But as ARCH_QCOM is
>>> enabled in the core defconfig, when I build smmuv3 as a module, I end
>>> up with a dependency on qcom-scm which gets built as an additional
>>> module. And it would be preferable to not require qcom modules to boot
>>> a tegra device.
>>
>> That just proves my point though - if you disable ARM_SMMU_QCOM in that
>> context then you've got a kernel which won't work properly on Qualcomm
>> platforms, so you may as well have just disabled ARCH_QCOM anyway. In
>> fact the latter is objectively better since it then would not break the
>> fundamental premise of "a core kernel that works across all supported
>> arch's" :/
> 
> I'm not sure this is entirely true. Google's GKI mandates a fixed core
> kernel Image. This has the minimal configs that can't be built as
> modules. Then each device build is supposed to build independent sets
> of modules via defconfig snippets that support the rest of the
> hardware. Then what gets booted on a device is a prebuilt core kernel
> image provided by Google, plus the modules built by the vendor. In
> this setup, qcom-scm and ARM_SMMU_QCOM are modules and not part of the
> core kernel. For a qcom target, arm_smmu_v3 would be built with
> ARM_SMMU_QCOM. But then any non-qcom target that needs arm_smmu_v3
> currently builds and deps qcom-scm. But there's no technical reason
> they need that dep.

There *is* a dependency, because when ARM_SMMU_QCOM is enabled and both 
ARM_SMMU=m and QCOM_SCM=m, arm-smmu.ko references symbols from 
qcom-scm.ko, so the module loader literally cannot load and dynamically 
link one without the other. As I said, you are welcome to do the work to 
try to relax that dependency somehow. What you cannot do is turn off 
ARM_SMMU_QCOM and functionally break ARCH_QCOM while still claiming to 
support ARCH_QCOM, because there is only one arm-smmu.ko - the fact that 
it's not built-in is immaterial, it's still effectively a "core" driver 
because it is shared by many different platforms.

Thanks,
Robin.

Re: [PATCH] iommu/arm: Allow disabling Qualcomm support in arm_smmu_v3

[Daniel Mentz]

On Mon, Mar 10, 2025 at 9:51 AM Aaron Kling <webgeek1234@gmail.com> wrote:
> I am working with the android kernel. The more recent setup enables a
> minimal setup of configs in a core kernel that works across all
> supported arch's, then requires further support to all be modules. I
> specifically am working with tegra devices. But as ARCH_QCOM is
> enabled in the core defconfig, when I build smmuv3 as a module, I end
> up with a dependency on qcom-scm which gets built as an additional
> module. And it would be preferable to not require qcom modules to boot
> a tegra device.

If you want to build arm_smmu_v3.ko, you'd have

# CONFIG_ARM_SMMU is not set
CONFIG_ARM_SMMU_V3=m

in your .config. I don't see how this would enable ARM_SMMU_QCOM or QCOM_SCM.

Re: [PATCH] iommu/arm: Allow disabling Qualcomm support in arm_smmu_v3

[Aaron Kling]

On Tue, Mar 11, 2025 at 12:55 PM Daniel Mentz <danielmentz@google.com> wrote:
>
> On Mon, Mar 10, 2025 at 9:51 AM Aaron Kling <webgeek1234@gmail.com> wrote:
> > I am working with the android kernel. The more recent setup enables a
> > minimal setup of configs in a core kernel that works across all
> > supported arch's, then requires further support to all be modules. I
> > specifically am working with tegra devices. But as ARCH_QCOM is
> > enabled in the core defconfig, when I build smmuv3 as a module, I end
> > up with a dependency on qcom-scm which gets built as an additional
> > module. And it would be preferable to not require qcom modules to boot
> > a tegra device.
>
> If you want to build arm_smmu_v3.ko, you'd have
>
> # CONFIG_ARM_SMMU is not set
> CONFIG_ARM_SMMU_V3=m
>
> in your .config. I don't see how this would enable ARM_SMMU_QCOM or QCOM_SCM.

I went and double checked my defconfig snippet and I have to
apologize. I put the wrong thing in the commit message and caused
confusion to myself and the entire discussion. This is what I've got:

# MMU
CONFIG_ARM_SMMU=m
# CONFIG_ARM_SMMU_QCOM is not set

Tegra186, tegra194, and tegra234 are supported by arm-smmu, not by
arm-smmu-v3. And these are the relevant archs I'm trying to work on.

Having the extra dep doesn't break anything, so worst case I continue
to carry the extra dep or this as a downstream patch. But I had to at
least try to decouple unnecessary dep.

Sincerely,
Aaron