On 19.2.2025 10.56, Michał Pecio wrote:
Hi,
+ /* Edge case, the TD wrapped around to the start segment. */
+ if (xhci_trb_virt_to_dma(td->end_seg, td->end_trb) < dma &&
+ dma < xhci_trb_virt_to_dma(td->start_seg, td->start_trb))
+ return NULL;
+ if (seg->dma <= dma && dma <= (seg->dma + TRB_SEGMENT_SIZE))
It should be strict inequality for the upper bound here.
Note that this wraparound case souldn't be happening (the driver avoids
moving enqueue into deq_seg to simplify ring expansion) so no amount of
testing will catch problems here, until maybe something changes one day.
+ return seg;
+ seg = seg->next;
+ }
The situation is tricky now, because we are either in start_seg and
end_seg is elsewhere or in start_seg->next and wraparound. But it looks
like the loop below will work OK for either case.
+ /* Loop through segment which don't contain the DMA address. */
+ while (dma < seg->dma || (seg->dma + TRB_SEGMENT_SIZE) <= dma) {
This condition looks like it could use the in_range() macro.
+ if (seg == td->end_seg)
+ return NULL;
+
+ seg = seg->next;
+ if (seg == td->start_seg)
+ return NULL;
I suppose this only happens if end_seg is not on the ring, fair enough.
+ }
Maybe a comment here? Something like:
* At this point seg contains the dma and either:
* a. start_seg != end_seg and seg can be anywhere
* b. start_seg == end_seg in wraparound case and seg != start_seg
Agreed, a comment here would help.
+ if (seg == td->start_seg) {
+ if (dma < xhci_trb_virt_to_dma(td->start_seg, td->start_trb))
+ return NULL;
+ } else if (seg == td->end_seg) {
+ if (xhci_trb_virt_to_dma(td->end_seg, td->end_trb) < dma)
+ return NULL;
+ }
+ return seg;
This should be corrent, but it's not something immediately obvious.
Not sure if this new implementation is really simpler than the old one.
I wonder if it wouldn't make sense to reorder this after the API change
(patch 4/4) to allow emergency revert if something unexpected shows up.
Had to draw several cases on paper to go through this new version.
But I might just be used to the old one
As for efficiency, those virt_to_dma translations aren't exactly free
and there are two. Maybe it could be faster to translate dma to virt
once and then compare. Sometimes also sizeof(*) < sizeof(dma_addr_t).
Agreed
dma_addr_t start_dma = xhci_trb_virt_to_dma(td->start_seg, td->start_trb);
dma_addr_t end_dma = xhci_trb_virt_to_dma(td->end_seg, td->end_trb);
comparisons will then be a lot easier to read with start_dma and end_dma
-Mathias
