[RFC PATCH v3 2/6] swiotlb: Add restricted DMA pool

[Christoph Hellwig]

> +#ifdef CONFIG_SWIOTLB
> +	struct io_tlb_mem	*dma_io_tlb_mem;
>  #endif

Please add a new config option for this code instead of always building
it when swiotlb is enabled.

> +static int swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t start,
> +				   size_t size)

Can you split the refactoring in swiotlb.c into one or more prep
patches?

> +static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
> +				    struct device *dev)
> +{
> +	struct io_tlb_mem *mem = rmem->priv;
> +	int ret;
> +
> +	if (dev->dma_io_tlb_mem)
> +		return -EBUSY;
> +
> +	if (!mem) {
> +		mem = kzalloc(sizeof(*mem), GFP_KERNEL);
> +		if (!mem)
> +			return -ENOMEM;

What is the calling convention here that allows for a NULL and non-NULL
private data?