Re: [PATCH v3 2/2] block-dma: properly take MMIO path

Leon Romanovsky
2025-10-27T07:57:47Z

On Mon, Oct 27, 2025 at 08:49:22AM +0100, Christoph Hellwig wrote:
> > + switch (iter.p2pdma.map) {
> > + case PCI_P2PDMA_MAP_BUS_ADDR:
> > + iod->flags |= IOD_DATA_P2P;
> > + break;
> > + case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> > + iod->flags |= IOD_DATA_MMIO;
> > + break;
> > + default:
> > + return BLK_STS_RESOURCE;
>
> I almost wonder if we should just the pci_p2pdma_map_type values into
> place. But that's a future cleanup, I'd rather get this going now.

I thought about it, but decided to use flags as more space efficient.
PCI_P2PDMA_MAP_BUS_ADDR == 3 and PCI_P2PDMA_MAP_THRU_HOST_BRIDGE == 4.
It means that will need to occupy 6 bits (extra u8 IOD field), while
this flags encoding takes only 4 bits (without extra IOD field).

>
> > +static inline bool blk_rq_dma_unmap(struct request *req, struct device *dma_dev,
> > + struct dma_iova_state *state, size_t mapped_len,
> > + enum pci_p2pdma_map_type map)
> > {
> > - if (is_p2p)
> > + if (map == PCI_P2PDMA_MAP_BUS_ADDR)
> > return true;
> >
> > if (dma_use_iova(state)) {
> > + unsigned int attrs = 0;
> > +
> > + if (map == PCI_P2PDMA_MAP_THRU_HOST_BRIDGE)
> > + attrs |= DMA_ATTR_MMIO;
> > +
> > dma_iova_destroy(dma_dev, state, mapped_len, rq_dma_dir(req),
> > - 0);
> > + attrs);
>
> The only thing in req that is used now is the data directrion. I'd be
> almost tempted to just pass that and lift this to dma-mapping.h.
>
> But I guess we could just do that in a follow on to not drag in
> another subsystem.
>
> Otherwise looks good:
>
> Reviewed-by: Christoph Hellwig <hch@lst.de>

Thanks