Re: [RFC PATCH v2 06/11] dma-direct: Support PCI P2PDMA pages in dma-direct map_sg

Robin Murphy
2021-03-12T15:52:13Z

On 2021-03-11 23:31, 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.
>
> SGL segments that contain PCI bus addresses are marked with
> sg_mark_pci_p2pdma() and are ignored when unmapped.
>
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> ---
> kernel/dma/direct.c | 35 ++++++++++++++++++++++++++++++++---
> kernel/dma/mapping.c | 13 ++++++++++---
> 2 files changed, 42 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 002268262c9a..f326d32062dd 100644
> --- a/kernel/dma/direct.c
> +++ 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"
>
> /*
> @@ -387,19 +388,47 @@ void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sgl,
> struct scatterlist *sg;
> int i;
>
> - for_each_sg(sgl, sg, nents, i)
> + for_each_sg(sgl, sg, nents, i) {
> + if (sg_is_pci_p2pdma(sg))
> + continue;
> +
> dma_direct_unmap_page(dev, sg->dma_address, sg_dma_len(sg), dir,
> attrs);
> + }
> }
> #endif
>
> int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
> enum dma_data_direction dir, unsigned long attrs)
> {
> - int i;
> + struct dev_pagemap *pgmap = NULL;
> + int i, map = -1, ret = 0;
> struct scatterlist *sg;
> + u64 bus_off;
>
> for_each_sg(sgl, sg, nents, i) {
> + if (is_pci_p2pdma_page(sg_page(sg))) {
> + if (sg_page(sg)->pgmap != pgmap) {
> + pgmap = sg_page(sg)->pgmap;
> + map = pci_p2pdma_dma_map_type(dev, pgmap);
> + bus_off = pci_p2pdma_bus_offset(sg_page(sg));
> + }
> +
> + if (map < 0) {
> + sg->dma_address = DMA_MAPPING_ERROR;
> + ret = -EREMOTEIO;
> + goto out_unmap;
> + }
> +
> + if (map) {
> + sg->dma_address = sg_phys(sg) + sg->offset -
> + bus_off;
> + sg_dma_len(sg) = sg->length;
> + sg_mark_pci_p2pdma(sg);
> + continue;
> + }
> + }
> +
> sg->dma_address = dma_direct_map_page(dev, sg_page(sg),
> sg->offset, sg->length, dir, attrs);
> if (sg->dma_address == DMA_MAPPING_ERROR)
> @@ -411,7 +440,7 @@ int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
>
> out_unmap:
> dma_direct_unmap_sg(dev, sgl, i, dir, attrs | DMA_ATTR_SKIP_CPU_SYNC);
> - return 0;
> + return ret;
> }
>
> dma_addr_t dma_direct_map_resource(struct device *dev, phys_addr_t paddr,
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index b6a633679933..adc1a83950be 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -178,8 +178,15 @@ void dma_unmap_page_attrs(struct device *dev, dma_addr_t addr, size_t size,
> EXPORT_SYMBOL(dma_unmap_page_attrs);
>
> /*
> - * dma_maps_sg_attrs returns 0 on error and > 0 on success.
> - * It should never return a value < 0.
> + * dma_maps_sg_attrs returns 0 on any resource error and > 0 on success.
> + *
> + * If 0 is returned, the mapping can be retried and will succeed once
> + * sufficient resources are available.

That's not a guarantee we can uphold. Retrying forever in the vain hope
that a device might evolve some extra address bits, or a bounce buffer
might magically grow big enough for a gigantic mapping, isn't
necessarily the best idea.

> + *
> + * If there are P2PDMA pages in the scatterlist then this function may
> + * return -EREMOTEIO to indicate that the pages are not mappable by the
> + * device. In this case, an error should be returned for the IO as it
> + * will never be successfully retried.
> */
> int dma_map_sg_attrs(struct device *dev, struct scatterlist *sg, int nents,
> enum dma_data_direction dir, unsigned long attrs)
> @@ -197,7 +204,7 @@ int dma_map_sg_attrs(struct device *dev, struct scatterlist *sg, int nents,
> ents = dma_direct_map_sg(dev, sg, nents, dir, attrs);
> else
> ents = ops->map_sg(dev, sg, nents, dir, attrs);
> - BUG_ON(ents < 0);
> +

This scares me - I hesitate to imagine the amount of driver/subsystem
code out there that will see nonzero and merrily set off iterating a
negative number of segments, if we open the floodgates of allowing
implementations to return error codes here.

Robin.

> debug_dma_map_sg(dev, sg, nents, ents, dir);
>
> return ents;