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

John Hubbard
2021-05-02T23:28:19Z

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,