Re: [PATCH 1/6] lib/scatterlist: add sg_set_dma_addr() function

["Christian König" <christian.koenig@amd.com>]

Am 12.03.20 um 11:47 schrieb Christoph Hellwig:
> On Thu, Mar 12, 2020 at 11:31:35AM +0100, Christian König wrote:
> [SNIP]
>> I mean we could come up with a new structure for this, but to me that just
>> looks like reinventing the wheel. Especially since drivers need to be able
>> to handle both I/O to system memory and I/O to PCIe BARs.
> The structure for holding the struct page side of the scatterlist is
> called struct bio_vec, so far mostly used by the block and networking
> code.

Yeah, I'm aware of this.

> The structure for holding dma addresses doesn't really exist
> in a generic form, but would be an array of these structures:
>
> struct dma_sg {
> 	dma_addr_t	addr;
> 	u32		len;
> };

So the whole idea is to nuke scatterlist/sg_table in the long term and 
switch over to using bio_vec as input and dma_sg as output for a DMA 
mapping operation.

Is that correct? If yes I could live with that, but it makes my patchset 
much more complicated.

> Keeping them separate is important as most IOMMU drivers will return
> less entries than you can feed them.  E.g. if your input boundaries
> are 4k aligned you will usually just get a single IOVA entry back.
> I will soon also have a dma mapping interface that will take advantage
> of that fact.

Yeah, I noticed as well that this is not really well handled.

Thanks for the feedback,
Christian.
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