Re: [PATCH v3 4/9] dma: swiotlb: track pool encryption state and honor DMA_ATTR_CC_SHARED

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Apr 27, 2026 at 11:25:04AM +0530, Aneesh Kumar K.V (Arm) wrote:
> @@ -1408,6 +1429,17 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
>  	if (cc_platform_has(CC_ATTR_MEM_ENCRYPT))
>  		pr_warn_once("Memory encryption is active and system is using DMA bounce buffers\n");
>  
> +	/*
> +	 * if we are trying to swiotlb map a decrypted paddr or the paddr is encrypted
> +	 * but the device is forcing decryption, use decrypted io_tlb_mem
> +	 */
> +	if ((attrs & DMA_ATTR_CC_SHARED) ||
> +	    (!(attrs & DMA_ATTR_CC_SHARED) && force_dma_unencrypted(dev)))
> +		require_decrypted = true;

Nit: just this should do:

	if ((attrs & DMA_ATTR_CC_SHARED) || force_dma_unencrypted(dev))

> +	if (require_decrypted != mem->decrypted)
> +		return (phys_addr_t)DMA_MAPPING_ERROR;

I wonder whether io_tlb_mem should store the attrs that were used when
created (just DMA_ATTR_CC_SHARED for now) and use that to check here. In
patch 7, this hunk in swiotlb_map() confused me:

	if (dev->dma_io_tlb_mem->decrypted) {
		dma_addr = phys_to_dma_unencrypted(dev, swiotlb_addr);
		attrs |= DMA_ATTR_CC_SHARED;
	} else {
		dma_addr = phys_to_dma_encrypted(dev, swiotlb_addr);
	}

as I thought we'd not update the attributes on the streaming API path.
But what you meant here is for dma_capable() to be checked against the
device with the actual io_tlb_mem attributes.

Anyway, the new swiotlb_tbl_map_single() rejects kmalloc-minalign
bouncing if the device is private while the bounce buffer is shared.
Unlikely we'll need such bouncing if the devices are coherent but it's
good as a safety check.

-- 
Catalin