Re: [PATCH 31/33] dma-direct: reject too small dma masks

Robin Murphy
2018-01-10T11:49:34Z

On 10/01/18 08:00, Christoph Hellwig wrote:
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> include/linux/dma-direct.h | 1 +
> lib/dma-direct.c | 19 +++++++++++++++++++
> 2 files changed, 20 insertions(+)
>
> diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
> index 4788bf0bf683..bcdb1a3e4b1f 100644
> --- a/include/linux/dma-direct.h
> +++ b/include/linux/dma-direct.h
> @@ -42,5 +42,6 @@ void *dma_direct_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
> gfp_t gfp, unsigned long attrs);
> void dma_direct_free(struct device *dev, size_t size, void *cpu_addr,
> dma_addr_t dma_addr, unsigned long attrs);
> +int dma_direct_supported(struct device *dev, u64 mask);
>
> #endif /* _LINUX_DMA_DIRECT_H */
> diff --git a/lib/dma-direct.c b/lib/dma-direct.c
> index 784a68dfdbe3..40b1f92f2214 100644
> --- a/lib/dma-direct.c
> +++ b/lib/dma-direct.c
> @@ -122,6 +122,24 @@ static int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl,
> return nents;
> }
>
> +int dma_direct_supported(struct device *dev, u64 mask)
> +{
> +#ifdef CONFIG_ZONE_DMA
> + if (mask < DMA_BIT_MASK(ARCH_ZONE_DMA_BITS))
> + return 0;
> +#else
> + /*
> + * Because 32-bit DMA masks are so common we expect every architecture
> + * to be able to satisfy them - either by not supporting more physical
> + * memory, or by providing a ZONE_DMA32. If neither is the case, the
> + * architecture needs to use an IOMMU instead of the direct mapping.
> + */
> + if (mask < DMA_BIT_MASK(32))
> + return 0;

Do you think it's worth the effort to be a little more accommodating
here? i.e.:

return dma_max_pfn(dev) >= max_pfn;

We seem to have a fair few 28-31 bit masks for older hardware which
probably associates with host systems packing equivalently small amounts
of RAM.

Otherwise though,

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

Robin.

> +#endif
> + return 1;
> +}
> +
> static int dma_direct_mapping_error(struct device *dev, dma_addr_t dma_addr)
> {
> return dma_addr == DIRECT_MAPPING_ERROR;
> @@ -132,6 +150,7 @@ const struct dma_map_ops dma_direct_ops = {
> .free = dma_direct_free,
> .map_page = dma_direct_map_page,
> .map_sg = dma_direct_map_sg,
> + .dma_supported = dma_direct_supported,
> .mapping_error = dma_direct_mapping_error,
> };
> EXPORT_SYMBOL(dma_direct_ops);
>