Re: [RFC][PATCH] dma-mapping: align default segment_boundary_mask with dma_mask

[Christoph Hellwig]

On Mon, Mar 16, 2020 at 12:12:08PM +0000, Robin Murphy wrote:
> On 2020-03-14 12:00 am, Nicolin Chen wrote:
>> More and more drivers set dma_masks above DMA_BIT_MAKS(32) while
>> only a handful of drivers call dma_set_seg_boundary(). This means
>> that most drivers have a 4GB segmention boundary because DMA API
>> returns DMA_BIT_MAKS(32) as a default value, though they might be
>> able to handle things above 32-bit.
>
> Don't assume the boundary mask and the DMA mask are related. There do exist 
> devices which can DMA to a 64-bit address space in general, but due to 
> descriptor formats/hardware design/whatever still require any single 
> transfer not to cross some smaller boundary. XHCI is 64-bit yet requires 
> most things not to cross a 64KB boundary. EHCI's 64-bit mode is an example 
> of the 4GB boundary (not the best example, admittedly, but it undeniably 
> exists).

Yes, which is what the boundary is for.  But why would we default to
something restrictive by default even if the driver didn't ask for it?