[PATCH] iommu/arm-smmu-v3: Skip prefetch cmd for MMU-600, MMU-700

[Daniel Mentz]

On Wed, May 15, 2024 at 8:29 AM Robin Murphy <robin.murphy at arm.com> wrote:
>
> Hi Daniel,
>
> On 2024-05-13 10:54 pm, Daniel Mentz wrote:
> > Arm CoreLink MMU-600 and MMU-700 both silently ignore the
> > CMD_PREFETCH_CONFIG command. Let's not waste cycles adding this command to
> > the command queue if it is ignored anyway.
>
> It is true that our implementations don't do anything with prefetch
> commands so far, however this is a valid architectural behaviour, and
> the intent of the quirk is really for buggy implementations where the
> commands erroneously fault.
>
> TBH if saving cycles in arm_smmu_attach_dev() matters to you, then
> you're probably not going to like what's now queued in next that makes
> it absurdly overcomplicated with loads of indirect function calls...

Hi Robin,

That's fair. Given the low frequency with which arm_smmu_attach_dev()
is called, it might not be worth the added complexity. Thanks for the
feedback, though.

>
> Thanks,
> Robin.
>
> > Signed-off-by: Daniel Mentz <danielmentz at google.com>
> > ---
> >   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > index aa62f0ecd053..98f1c4b44ef3 100644
> > --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > @@ -3684,6 +3684,8 @@ static void arm_smmu_device_iidr_probe(struct arm_smmu_device *smmu)
> >       case IIDR_IMPLEMENTER_ARM:
> >               switch (productid) {
> >               case IIDR_PRODUCTID_ARM_MMU_600:
> > +                     /* MMU-600 silently ignores CMD_PREFETCH_CONFIG */
> > +                     smmu->options |= ARM_SMMU_OPT_SKIP_PREFETCH;
> >                       /* Arm erratum 1076982 */
> >                       if (variant == 0 && revision <= 2)
> >                               smmu->features &= ~ARM_SMMU_FEAT_SEV;
> > @@ -3692,6 +3694,8 @@ static void arm_smmu_device_iidr_probe(struct arm_smmu_device *smmu)
> >                               smmu->features &= ~ARM_SMMU_FEAT_NESTING;
> >                       break;
> >               case IIDR_PRODUCTID_ARM_MMU_700:
> > +                     /* MMU-700 silently ignores CMD_PREFETCH_CONFIG */
> > +                     smmu->options |= ARM_SMMU_OPT_SKIP_PREFETCH;
> >                       /* Arm erratum 2812531 */
> >                       smmu->features &= ~ARM_SMMU_FEAT_BTM;
> >                       smmu->options |= ARM_SMMU_OPT_CMDQ_FORCE_SYNC;