On 4/8/21 10:01 AM, Logan Gunthorpe wrote:

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