Re: [PATCH v3 06/20] dma-direct: support PCI P2PDMA pages in dma-direct map_sg

[Jason Gunthorpe]

On Thu, Sep 16, 2021 at 05:40:46PM -0600, Logan Gunthorpe wrote:
> Add PCI P2PDMA support for dma_direct_map_sg() so that it can map
> PCI P2PDMA pages directly without a hack in the callers. This allows
> for heterogeneous SGLs that contain both P2PDMA and regular pages.
> 
> A P2PDMA page may have three possible outcomes when being mapped:
>   1) If the data path between the two devices doesn't go through the
>      root port, then it should be mapped with a PCI bus address
>   2) If the data path goes through the host bridge, it should be mapped
>      normally, as though it were a CPU physical address
>   3) It is not possible for the two devices to communicate and thus
>      the mapping operation should fail (and it will return -EREMOTEIO).
> 
> SGL segments that contain PCI bus addresses are marked with
> sg_dma_mark_pci_p2pdma() and are ignored when unmapped.
> 
> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
>  kernel/dma/direct.c | 44 ++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 38 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 4c6c5e0635e3..fa8317e8ff44 100644
> +++ b/kernel/dma/direct.c
> @@ -13,6 +13,7 @@
>  #include <linux/vmalloc.h>
>  #include <linux/set_memory.h>
>  #include <linux/slab.h>
> +#include <linux/pci-p2pdma.h>
>  #include "direct.h"
>  
>  /*
> @@ -421,29 +422,60 @@ void dma_direct_sync_sg_for_cpu(struct device *dev,
>  		arch_sync_dma_for_cpu_all();
>  }
>  
> +/*
> + * Unmaps segments, except for ones marked as pci_p2pdma which do not
> + * require any further action as they contain a bus address.
> + */
>  void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sgl,
>  		int nents, enum dma_data_direction dir, unsigned long attrs)
>  {
>  	struct scatterlist *sg;
>  	int i;
>  
> -	for_each_sg(sgl, sg, nents, i)
> -		dma_direct_unmap_page(dev, sg->dma_address, sg_dma_len(sg), dir,
> -			     attrs);
> +	for_each_sg(sgl, sg, nents, i) {
> +		if (sg_is_dma_pci_p2pdma(sg)) {
> +			sg_dma_unmark_pci_p2pdma(sg);
> +		} else  {
> +			dma_direct_unmap_page(dev, sg->dma_address,
> +					      sg_dma_len(sg), dir, attrs);
> +		}

If the main usage of this SGL bit is to indicate if it has been DMA
mapped, or not, I think it should be renamed to something clearer.

p2pdma is being used for lots of things now, it feels very
counter-intuitive that P2PDMA pages are not flagged with
something called sg_is_dma_pci_p2pdma().

How about sg_is_dma_unmapped_address() ?
>  
>  	for_each_sg(sgl, sg, nents, i) {
> +		if (is_pci_p2pdma_page(sg_page(sg))) {
> +			map = pci_p2pdma_map_segment(&p2pdma_state, dev, sg);
> +			switch (map) {
> +			case PCI_P2PDMA_MAP_BUS_ADDR:
> +				continue;
> +			case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> +				/*
> +				 * Mapping through host bridge should be
> +				 * mapped normally, thus we do nothing
> +				 * and continue below.
> +				 */
> +				break;
> +			default:
> +				ret = -EREMOTEIO;
> +				goto out_unmap;
> +			}
> +		}
> +
>  		sg->dma_address = dma_direct_map_page(dev, sg_page(sg),
>  				sg->offset, sg->length, dir, attrs);

dma_direct_map_page() can trigger swiotlb and I didn't see this series
dealing with that?

It would probably be fine for now to fail swiotlb_map() for p2p pages?

Jason