Re: [PATCH 20/21] ARM: dma-mapping: split out arch_dma_mark_clean() helper

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

On 2023-03-27 13:13, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> The arm version of the arch_sync_dma_for_cpu() function annotates pages as
> PG_dcache_clean after a DMA, but no other architecture does this here. On
> ia64, the same thing is done in arch_sync_dma_for_cpu(), so it makes sense
> to use the same hook in order to have identical arch_sync_dma_for_cpu()
> semantics as all other architectures.
> 
> Splitting this out has multiple effects:
> 
>   - for dma-direct, this now gets called after arch_sync_dma_for_cpu()
>     for DMA_FROM_DEVICE mappings, but not for DMA_BIDIRECTIONAL. While
>     it would not be harmful to keep doing it for bidirectional mappings,
>     those are apparently not used in any callers that care about the flag.
> 
>   - Since arm has its own dma-iommu abstraction, this now also needs to
>     call the same function, so the calls are added there to mirror the
>     dma-direct version.
> 
>   - Like dma-direct, the dma-iommu version now marks the dcache clean
>     for both coherent and noncoherent devices after a DMA, but it only
>     does this for DMA_FROM_DEVICE, not DMA_BIDIRECTIONAL.
> 
> [ HELP NEEDED: can anyone confirm that it is a correct assumption
>    on arm that a cache-coherent device writing to a page always results
>    in it being in a PG_dcache_clean state like on ia64, or can a device
>    write directly into the dcache?]

In AMBA at least, if a snooping write hits in a cache then the data is 
most likely going to get routed directly into that cache. If it has 
write-back write-allocate attributes it could also land in any cache 
along its normal path to RAM; it wouldn't have to go all the way.

Hence all the fun we have where treating a coherent device as 
non-coherent can still be almost as broken as the other way round :)

Cheers,
Robin.

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>   arch/arm/Kconfig          |  1 +
>   arch/arm/mm/dma-mapping.c | 71 +++++++++++++++++++++++----------------
>   2 files changed, 43 insertions(+), 29 deletions(-)
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index e24a9820e12f..125d58c54ab1 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -7,6 +7,7 @@ config ARM
>   	select ARCH_HAS_BINFMT_FLAT
>   	select ARCH_HAS_CURRENT_STACK_POINTER
>   	select ARCH_HAS_DEBUG_VIRTUAL if MMU
> +	select ARCH_HAS_DMA_MARK_CLEAN if MMU
>   	select ARCH_HAS_DMA_WRITE_COMBINE if !ARM_DMA_MEM_BUFFERABLE
>   	select ARCH_HAS_ELF_RANDOMIZE
>   	select ARCH_HAS_FORTIFY_SOURCE
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index cc702cb27ae7..b703cb83d27e 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -665,6 +665,28 @@ static void dma_cache_maint(phys_addr_t paddr,
>   	} while (left);
>   }
>   
> +/*
> + * Mark the D-cache clean for these pages to avoid extra flushing.
> + */
> +void arch_dma_mark_clean(phys_addr_t paddr, size_t size)
> +{
> +	unsigned long pfn = PFN_UP(paddr);
> +	unsigned long off = paddr & (PAGE_SIZE - 1);
> +	size_t left = size;
> +
> +	if (size < PAGE_SIZE)
> +		return;
> +
> +	if (off)
> +		left -= PAGE_SIZE - off;
> +
> +	while (left >= PAGE_SIZE) {
> +		struct page *page = pfn_to_page(pfn++);
> +		set_bit(PG_dcache_clean, &page->flags);
> +		left -= PAGE_SIZE;
> +	}
> +}
> +
>   static bool arch_sync_dma_cpu_needs_post_dma_flush(void)
>   {
>   	if (IS_ENABLED(CONFIG_CPU_V6) ||
> @@ -715,24 +737,6 @@ void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size,
>   		outer_inv_range(paddr, paddr + size);
>   		dma_cache_maint(paddr, size, dmac_inv_range);
>   	}
> -
> -	/*
> -	 * Mark the D-cache clean for these pages to avoid extra flushing.
> -	 */
> -	if (dir != DMA_TO_DEVICE && size >= PAGE_SIZE) {
> -		unsigned long pfn = PFN_UP(paddr);
> -		unsigned long off = paddr & (PAGE_SIZE - 1);
> -		size_t left = size;
> -
> -		if (off)
> -			left -= PAGE_SIZE - off;
> -
> -		while (left >= PAGE_SIZE) {
> -			struct page *page = pfn_to_page(pfn++);
> -			set_bit(PG_dcache_clean, &page->flags);
> -			left -= PAGE_SIZE;
> -		}
> -	}
>   }
>   
>   #ifdef CONFIG_ARM_DMA_USE_IOMMU
> @@ -1294,6 +1298,17 @@ static int arm_iommu_map_sg(struct device *dev, struct scatterlist *sg,
>   	return -EINVAL;
>   }
>   
> +static void arm_iommu_sync_dma_for_cpu(phys_addr_t phys, size_t len,
> +				       enum dma_data_direction dir,
> +				       bool dma_coherent)
> +{
> +	if (!dma_coherent)
> +		arch_sync_dma_for_cpu(phys, s->length, dir);
> +
> +	if (dir == DMA_FROM_DEVICE)
> +		arch_dma_mark_clean(phys, s->length);
> +}
> +
>   /**
>    * arm_iommu_unmap_sg - unmap a set of SG buffers mapped by dma_map_sg
>    * @dev: valid struct device pointer
> @@ -1316,8 +1331,9 @@ static void arm_iommu_unmap_sg(struct device *dev,
>   		if (sg_dma_len(s))
>   			__iommu_remove_mapping(dev, sg_dma_address(s),
>   					       sg_dma_len(s));
> -		if (!dev->dma_coherent && !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> -			arch_sync_dma_for_cpu(sg_phys(s), s->length, dir);
> +		if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> +			arm_iommu_sync_dma_for_cpu(sg_phys(s), s->length, dir,
> +						   dev->dma_coherent);
>   	}
>   }
>   
> @@ -1335,12 +1351,9 @@ static void arm_iommu_sync_sg_for_cpu(struct device *dev,
>   	struct scatterlist *s;
>   	int i;
>   
> -	if (dev->dma_coherent)
> -		return;
> -
>   	for_each_sg(sg, s, nents, i)
> -		arch_sync_dma_for_cpu(sg_phys(s), s->length, dir);
> -
> +		arm_iommu_sync_dma_for_cpu(sg_phys(s), s->length, dir,
> +					   dev->dma_coherent);
>   }
>   
>   /**
> @@ -1425,9 +1438,9 @@ static void arm_iommu_unmap_page(struct device *dev, dma_addr_t handle,
>   	if (!iova)
>   		return;
>   
> -	if (!dev->dma_coherent && !(attrs & DMA_ATTR_SKIP_CPU_SYNC)) {
> +	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))
>   		phys = iommu_iova_to_phys(mapping->domain, handle);
> -		arch_sync_dma_for_cpu(phys, size, dir);
> +		arm_iommu_sync_dma_for_cpu(phys, size, dir, dev->dma_coherent);
>   	}
>   
>   	iommu_unmap(mapping->domain, iova, len);
> @@ -1497,11 +1510,11 @@ static void arm_iommu_sync_single_for_cpu(struct device *dev,
>   	struct dma_iommu_mapping *mapping = to_dma_iommu_mapping(dev);
>   	phys_addr_t phys;
>   
> -	if (dev->dma_coherent || !(handle & PAGE_MASK))
> +	if (!(handle & PAGE_MASK))
>   		return;
>   
>   	phys = iommu_iova_to_phys(mapping->domain, handle);
> -	arch_sync_dma_for_cpu(phys, size, dir);
> +	arm_iommu_sync_dma_for_cpu(phys, size, dir, dev->dma_coherent);
>   }
>   
>   static void arm_iommu_sync_single_for_device(struct device *dev,