Re: [PATCHv2 1/7] blk-mq-dma: move the bio and bvec_iter to blk_dma_iter

[Christoph Hellwig <hch@lst.de>]

On Mon, Jul 21, 2025 at 08:33:02PM -0600, Keith Busch wrote:
> On Mon, Jul 21, 2025 at 09:42:23AM +0200, Christoph Hellwig wrote:
> > On Sun, Jul 20, 2025 at 11:40:34AM -0700, Keith Busch wrote:
> > > From: Keith Busch <kbusch@kernel.org>
> > > 
> > > The req_iterator just happens to have a similar fields to what the dma
> > > iterator needs, but we're not necessarily iterating a bio_vec here. Have
> > > the dma iterator define its private fields directly. It also helps to
> > > remove eyesores like "iter->iter.iter".
> > 
> > Going back to this after looking at the later patches.  The level
> > for which this iter exists is only called blk_map_* as there is
> > nothing dma specific about, just mapping to physical addresses.
> > 
> > So maybe call it blk_map_iter instead?
> 
> But the structure yields "dma_addr_t" type values to the consumer.
> Whether those are physical addresses or remapped io virtual addresses,
> they're still used for direct memory access, so I think the name as-is
> is a pretty good fit.

There's two layers:

The lower layer implemented by blk_map_iter_next just yields a phys_vec.

The upper layer built on top does the dma mapping in case of the new
blk_rq_dma_map_iter_start / blk_rq_dma_map_iter_next API.  But in case of
the old __blk_rq_map_sg API it doesn't even directly do the DMA mapping
yet.  And some drivers at least historically used the blk_map_sg without
then doing a DMA mapping, either for MMIO or platform specific things
using physical address.  My hope was to eventually migrate them to use
blk_map_iter_next.