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@xxxxxxxxxxxx>
---
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,
