[PATCH] dma-mapping: treat dev->bus_dma_mask as a DMA limit

[Robin Murphy]

On 13/11/2019 4:13 pm, Nicolas Saenz Julienne wrote:
> Using a mask to represent bus DMA constraints has a set of limitations.
> The biggest one being it can only hold a power of two (minus one). The
> DMA mapping code is already aware of this and treats dev->bus_dma_mask
> as a limit. This quirk is already used by some architectures although
> still rare.
> 
> With the introduction of the Raspberry Pi 4 we've found a new contender
> for the use of bus DMA limits, as its PCIe bus can only address the
> lower 3GB of memory (of a total of 4GB). This is impossible to represent
> with a mask. To make things worse the device-tree code rounds non power
> of two bus DMA limits to the next power of two, which is unacceptable in
> this case.
> 
> In the light of this, rename dev->bus_dma_mask to dev->bus_dma_limit all
> over the tree and treat it as such. Note that dev->bus_dma_limit is
> meant to contain the higher accesible DMA address.

Neat, you win a "why didn't I do it that way in the first place?" :)

Looking at it without all the history of previous attempts, this looks 
entirely reasonable, and definitely a step in the right direction.

[...]
> diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
> index 5a7551d060f2..f18827cf96df 100644
> --- a/drivers/acpi/arm64/iort.c
> +++ b/drivers/acpi/arm64/iort.c
> @@ -1097,7 +1097,7 @@ void iort_dma_setup(struct device *dev, u64 *dma_addr, u64 *dma_size)
>   		 * Limit coherent and dma mask based on size
>   		 * retrieved from firmware.
>   		 */
> -		dev->bus_dma_mask = mask;
> +		dev->bus_dma_limit = mask;

Although this preserves the existing behaviour, as in of_dma_configure() 
we can do better here since we have the original address range to hand. 
I think it's worth keeping the ACPI and OF paths in sync for minor 
tweaks like this, rather than letting them diverge unnecessarily.

Otherwise, the rest looks OK to me - in principle we could store it as 
an exclusive limit such that we could then streamline the min_not_zero() 
tests to just min(mask, limit - 1), but that's probably too clever for 
its own good.

Robin.

>   		dev->coherent_dma_mask = mask;
>   		*dev->dma_mask = mask;
>   	}