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

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Message-ID: <d37ccb03-0d03-46ff-b62f-2fcb78263fe7@samsung.com>
Date: Mon, 28 Apr 2025 17:33:35 +0200
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: Christoph Hellwig <hch@....de>, Ian Abbott <abbotti@....co.uk>
Cc: linux-kernel@...r.kernel.org, Greg Kroah-Hartman
	<gregkh@...uxfoundation.org>, H Hartley Sweeten
	<hsweeten@...ionengravers.com>, Robin Murphy <robin.murphy@....com>,
	iommu@...ts.linux.dev
Subject: Re: [PATCH 4/4] comedi: allocate DMA coherent buffer as individual
 pages

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