Re: [PATCH 09/16] dma-direct: Support PCI P2PDMA pages in dma-direct map_sg

[John Hubbard <jhubbard@nvidia.com>]

On 4/8/21 10:01 AM, 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 | 25 ++++++++++++++++++++++---
>   1 file changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 002268262c9a..108dfb4ecbd5 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 +388,37 @@ void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sgl,

This routine now deserves a little bit of commenting, now that it is
doing less obvious things. How about something like this:

/*
  * Unmaps pages, except for PCI_P2PDMA pages, which were never mapped in the
  * first place. Instead of unmapping PCI_P2PDMA entries, simply remove the
  * SG_PCI_P2PDMA mark
  */
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)
> +	for_each_sg(sgl, sg, nents, i) {
> +		if (sg_is_pci_p2pdma(sg)) {
> +			sg_unmark_pci_p2pdma(sg);
> +			continue;
> +		}
> +
>   		dma_direct_unmap_page(dev, sg->dma_address, sg_dma_len(sg), dir,
>   			     attrs);
> +	}

The same thing can be achieved with fewer lines and a bit more clarity.
Can we please do it like this instead:

	for_each_sg(sgl, sg, nents, i) {
		if (sg_is_pci_p2pdma(sg))
			sg_unmark_pci_p2pdma(sg);
		else
			dma_direct_unmap_page(dev, sg->dma_address,
					      sg_dma_len(sg), dir, attrs);
	}


>   }
>   #endif
>   

Also here, a block comment for the function would be nice. How about
approximately this:

/*
  * Maps each SG segment. Returns the number of entries mapped, or 0 upon
  * failure. If any entry could not be mapped, then no entries are mapped.
  */

I'll stop complaining about the pre-existing return code conventions,
since by now you know what I was thinking of saying. :)

>   int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
>   		enum dma_data_direction dir, unsigned long attrs)
>   {
> -	int i;
> +	struct pci_p2pdma_map_state p2pdma_state = {};

Is it worth putting this stuff on the stack--is there a noticeable
performance improvement from caching the state? Because if it's
invisible, then simplicity is better. I suspect you're right, and that
it *is* worth it, but it's good to know for real.

>   	struct scatterlist *sg;
> +	int i, ret = 0;
>   
>   	for_each_sg(sgl, sg, nents, i) {
> +		if (is_pci_p2pdma_page(sg_page(sg))) {
> +			ret = pci_p2pdma_map_segment(&p2pdma_state, dev, sg,
> +						     attrs);
> +			if (ret < 0) {
> +				goto out_unmap;
> +			} else if (ret) {
> +				ret = 0;
> +				continue;

Is this a bug? If neither of those "if" branches fires (ret == 0), then
the code (probably unintentionally) falls through and continues on to
attempt to call dma_direct_map_page()--despite being a PCI_P2PDMA page!

See below for suggestions:

> +			}
> +		}
> +
>   		sg->dma_address = dma_direct_map_page(dev, sg_page(sg),
>   				sg->offset, sg->length, dir, attrs);
>   		if (sg->dma_address == DMA_MAPPING_ERROR)

This is another case in which "continue" is misleading and not as good
as "else". Because unless I'm wrong above, you really only want to take
one path *or* the other.

Also, the "else if (ret)" can be simplified to just setting ret = 0
unconditionally.

Given all that, here's a suggested alternative, which is both shorter
and clearer, IMHO:

	for_each_sg(sgl, sg, nents, i) {
		if (is_pci_p2pdma_page(sg_page(sg))) {
			ret = pci_p2pdma_map_segment(&p2pdma_state, dev, sg,
						     attrs);
			if (ret < 0)
				goto out_unmap;
			else
				ret = 0;
		} else {
			sg->dma_address = dma_direct_map_page(dev, sg_page(sg),
					sg->offset, sg->length, dir, attrs);
			if (sg->dma_address == DMA_MAPPING_ERROR)
				goto out_unmap;
			sg_dma_len(sg) = sg->length;
		}
	}

thanks,
-- 
John Hubbard
NVIDIA

> @@ -411,7 +430,7 @@ 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,
>