Re: [PATCH v3 0/2] swiotlb: allocate padding slots if necessary

[Will Deacon]

On Fri, Mar 22, 2024 at 06:51:38PM +0100, Petr Tesařík wrote:
> On Fri, 22 Mar 2024 15:09:41 +0000
> Will Deacon <will@xxxxxxxxxx> wrote:
> 
> > Hi Petr,
> > 
> > On Thu, Mar 21, 2024 at 06:19:00PM +0100, Petr Tesarik wrote:
> > > From: Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx>
> > > 
> > > If the allocation alignment is bigger than IO_TLB_SIZE and min_align_mask
> > > covers some bits in the original address between IO_TLB_SIZE and
> > > alloc_align_mask, preserve these bits by allocating additional padding
> > > slots before the actual swiotlb buffer.  
> > 
> > Thanks for fixing this! I was out at a conference last week, so I didn't
> > get very far with it myself, but I ended up in a pickle trying to avoid
> > extending 'struct io_tlb_slot'. Your solution is much better than the
> > crazy avenue I started going down...
> > 
> > With your changes, can we now simplify swiotlb_align_offset() to ignore
> > dma_get_min_align_mask() altogether and just:
> > 
> > 	return addr & (IO_TLB_SIZE - 1);
> 
> I have also thought about this but I don't think it's right. If we
> removed dma_get_min_align_mask() from swiotlb_align_offset(), we would
> always ask to preserve the lowest IO_TLB_SHIFT bits. This may cause
> less efficient use of the SWIOTLB.
> 
> For example, if a device does not specify any min_align_mask, it is
> presumably happy with any buffer alignment, so SWIOTLB may allocate at
> the beginning of a slot, like here:
> 
> orig_addr   |      ++|++      |
> tlb_addr    |++++    |        |
> 
> Without dma_get_min_align_mask() in swiotlb_align_offset(), it would
> have to allocate two mostly-empty slots:
> 
> tlb_addr    |      ++|++      |
> 
> where:
>   | mark a multiple of IO_TLB_SIZE (in physical address space)
>   + used memory
>     free memory

Thanks for the patient explanation. I'd got so caught up with the DMA
alignment mask that I forgot the usual case where it's not specified at
all!

Will