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

[Robin Murphy <robin.murphy@arm.com>]

On 2021-02-26 14:03, Nicolas Saenz Julienne wrote:
> Some arm SMMU implementations might sit on a bus that doesn't support
> 64bit memory accesses. In that case default to using hi_lo_{readq,
> writeq}() and BUG if such platform tries to use AArch64 formats as they
> rely on writeq()'s atomicity.
> 
> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
> ---
>   drivers/iommu/arm/arm-smmu/arm-smmu.c | 9 +++++++++
>   drivers/iommu/arm/arm-smmu/arm-smmu.h | 9 +++++++--
>   2 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> index d8c6bfde6a61..239ff42b20c3 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> @@ -1889,6 +1889,15 @@ static int arm_smmu_device_cfg_probe(struct arm_smmu_device *smmu)
>   			smmu->features |= ARM_SMMU_FEAT_FMT_AARCH64_64K;
>   	}
>   
> +	/*
> +	 * 64bit accesses not possible through the interconnect, AArch64
> +	 * formats depend on it.
> +	 */
> +	BUG_ON(!dev_64bit_mmio_supported(smmu->dev) &&
> +	       smmu->features & (ARM_SMMU_FEAT_FMT_AARCH64_4K |
> +				ARM_SMMU_FEAT_FMT_AARCH64_16K |
> +				ARM_SMMU_FEAT_FMT_AARCH64_64K));

No. Crashing the kernel in a probe routine which is free to fail is 
unacceptable either way, but guaranteeing failure in the case that the 
workaround *would* be required is doubly so.

Basically, this logic is backwards - if you really wanted to handle it 
generically, this would be the point at which you'd need to actively 
suppress all the detected hardware features which depend on 64-bit 
atomicity, not complain about them.

> +
>   	if (smmu->impl && smmu->impl->cfg_probe) {
>   		ret = smmu->impl->cfg_probe(smmu);
>   		if (ret)
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.h b/drivers/iommu/arm/arm-smmu/arm-smmu.h
> index d2a2d1bc58ba..997d13a21717 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu.h
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.h
> @@ -477,15 +477,20 @@ static inline void arm_smmu_writel(struct arm_smmu_device *smmu, int page,
>   {
>   	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);

As Arnd pointed out, this is in completely the wrong place. Also, in 
general it doesn't work if the implementation already needs a hook to 
filter or override register accesses for any other reason. TBH I'm not 
convinced that this isn't *more* of a mess than handling it on a 
SoC-specific basis...

Robin.

>   }
>   
>   static inline u64 arm_smmu_readq(struct arm_smmu_device *smmu, int page, int offset)
>   {
>   	if (smmu->impl && unlikely(smmu->impl->read_reg64))
>   		return smmu->impl->read_reg64(smmu, page, offset);
> -	return readq_relaxed(arm_smmu_page(smmu, page) + offset);
> +	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);
>   }
>   
>   static inline void arm_smmu_writeq(struct arm_smmu_device *smmu, int page,
>