[PATCH] arm64: avoid increasing DMA masks above what hardware supports

[Nikita Yushchenko]

>> +	/*
>> +	 * we don't yet support buses that have a non-zero mapping.
>> +	 *  Let's hope we won't need it
>> +	 */
>> +	WARN_ON(dma_base != 0);
> 
> I believe we now accomodate the bus remap bits on BCM2837 as a DMA
> offset, so unfortunately I think this is no longer true.

Arnd, this check is from you. Any updates? Perhaps this check can be
just dropped?

In swiotlb code, dma address (i.e. with offset already applied) is
checked against mask.  Not sure what 'dma_base' means in iommu case.

>> +	/*
>> +	 * Whatever the parent bus can set. A device must not set
>> +	 * a DMA mask larger than this.
>> +	 */
>> +	dev->archdata.parent_dma_mask = size - 1;
> 
> This will effectively constrain *all* DMA masks to be 32-bit, since for
> 99% of devices we're going to see a size derived from the default mask
> passed in here. I worry that that's liable to lead to performance and
> stability regressions

That was exactly my concern when I first tried to address this issue. My
first attempt was to alter very locally exact configuration where
problem shows, while ensuring that everything else stays as is. See
https://lkml.org/lkml/2016/12/29/218

But looks like people want a generic solution.

> I reckon the easiest way forward would be to pass in some flag to
> arch_setup_dma_ops to indicate whether it's an explicitly-configured
> range or not - then simply initialising parent_dma_mask to ~0 for the
> default case *should* keep things working as before.

Currently only arm, arm64 and mips define arch_setup_dma_ops().
Mips version only checks 'coherent' argument, 'size' is used only by arm
and arm64.

Maybe move setting the default from caller to callee?
I.e. pass size=0 if no explicit information exists, and let architecture
handle that?