Re: [RFC 09/13] iommu/arm-smmu: Make use of dev_64bit_mmio_supported()

Hi Arnd, thanks for the reviews!

On Tue, 2021-03-02 at 10:32 +0100, Arnd Bergmann wrote:

On Fri, Feb 26, 2021 at 3:03 PM Nicolas Saenz Julienne
[off-list ref] wrote:

quoted

        if (smmu->impl && unlikely(smmu->impl->write_reg))
                smmu->impl->write_reg(smmu, page, offset, val);
-       else
+       else if (dev_64bit_mmio_supported(smmu->dev))
                writel_relaxed(val, arm_smmu_page(smmu, page) + offset);
+       else
+               hi_lo_writeq_relaxed(val, arm_smmu_page(smmu, page) + offset);
 }

This is a writel_relaxed(), not a writeq_relaxed(), so I suppose you don't
have to change it at all.

Yes, that was silly of me. I was worrying about the semantics of the whole
thing, and missed basic stuff like this.

quoted

+       else if (dev_64bit_mmio_supported(smmu->dev))
+               return readq_relaxed(arm_smmu_page(smmu, page) + offset);
+       else
+               return hi_lo_readq_relaxed(arm_smmu_page(smmu, page) + offset);
}

I see this pattern repeat across multiple drivers. I think Christoph
had originally
suggested folding the if/else logic into the writel_relaxed() that is defined in
include/linux/io-64-nonatomic-hi-lo.h, but of course that doesn't work if you
need to pass a device pointer.

It might still make sense to have another wrapper in that same file though,
something like

static inline hi_lo_writeq_relaxed_if_possible(struct device *dev, __u64 val,
                    volatile void __iomem *addr)
{
       if (dev_64bit_mmio_supported(smmu->dev)) {
              readq_relaxed(arm_smmu_page(smmu, page) + offset);
       } else {
               writel_relaxed(val >> 32, addr + 4);
               writel_relaxed(val, addr);
       }
}

I like the idea. I'll try to integrate it into the next revision.

Regards,
Nicolas