Re: [PATCH 4/4] comedi: allocate DMA coherent buffer as individual pages

[Marek Szyprowski <m.szyprowski@samsung.com>]

On 28.04.2025 14:56, Christoph Hellwig wrote:
> On Tue, Apr 15, 2025 at 12:35:59PM +0100, Ian Abbott wrote:
>> +			vma->vm_start = start;
>> +			vma->vm_end = start + PAGE_SIZE;
>> +			retval = dma_mmap_coherent(bm->dma_hw_dev, vma,
>> +						   buf->virt_addr,
>> +						   buf->dma_addr, PAGE_SIZE);
> I'm not  fan of the vm_start/vm_end manipulation, but I've seen it in
> other places.  In a perfect world we'd have a dma_mmap_coherent_offset
> or similar helper that encapsulates it, and then maybe later replace
> that hack with passing on the offset.

Indeed the dma_mmap_*() makes too many assumptions about the vma. The 
case You mentioned is probably in drivers/infiniband/hw/hfi1/file_ops.c 
but I also see that the vma->vm_pgoff is being adjusted before most 
dma_mmap_*() calls, which proves that the current API is somehow 
limited. It would be great to fix this too while touching the 
dma_mmap_attrs() API.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland