Re: [PATCH] iommu/arm-smmu: Report USF more clearly

[Doug Anderson <dianders@chromium.org>]

Hi,

On Fri, Sep 13, 2019 at 4:48 AM Robin Murphy <robin.murphy@arm.com> wrote:
>
> Although CONFIG_ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT is a welcome tool
> for smoking out inadequate firmware, the failure mode is non-obvious
> and can be confusing for end users. Add some special-case reporting of
> Unidentified Stream Faults to help clarify this particular symptom.
>
> CC: Douglas Anderson <dianders@chromium.org>

nit that I believe that "Cc" (lowercase 2nd c) is correct.

> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>  drivers/iommu/arm-smmu.c | 5 +++++
>  drivers/iommu/arm-smmu.h | 2 ++
>  2 files changed, 7 insertions(+)
>
> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> index b7cf24402a94..76ac8c180695 100644
> --- a/drivers/iommu/arm-smmu.c
> +++ b/drivers/iommu/arm-smmu.c
> @@ -499,6 +499,11 @@ static irqreturn_t arm_smmu_global_fault(int irq, void *dev)
>         dev_err_ratelimited(smmu->dev,
>                 "\tGFSR 0x%08x, GFSYNR0 0x%08x, GFSYNR1 0x%08x, GFSYNR2 0x%08x\n",
>                 gfsr, gfsynr0, gfsynr1, gfsynr2);
> +       if (IS_ENABLED(CONFIG_ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT) &&
> +           (gfsr & sGFSR_USF))
> +               dev_err_ratelimited(smmu->dev,
> +                       "Stream ID %hu may not be described by firmware, try booting with \"arm-smmu.disable_bypass=0\"\n",
> +                       (u16)gfsynr1);

In general it seems like a sane idea to surface an error like this.  I
guess a few nits:

1. "By firmware" might be a bit misleading.  In most cases I'm aware
of the problem is in the device tree that was bundled together with
the kernel.  If there are actually cases where firmware has baked in a
device tree and it got this wrong then we might want to spend time
figuring out what to do about it.

2. Presumably booting with "arm-smmu.disable_bypass=0" is in most
cases the least desirable option available.  I always consider kernel
command line parameters as something of a last resort for
configuration and would only be something that and end user might do
if they were given a kernel compiled by someone else (like if someone
where taking a prebuilt Linux distro and trying to install it onto a
generic PC).  Are you seeing cases where this is happening?  If people
are compiling their own kernel I'd argue that telling them to set
"CONFIG_ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT" to "no" is much better
than trying to jam a command line option on.  Command line options
don't scale well.

3. Any chance you could make it more obvious that this change is
undesirable and a last resort?  AKA:

"Stream ID x blocked for security reasons; allow anyway by booting
with arm-smmu.disable_bypass=0"

-Doug

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