[PATCH 5/4 RFC] An alternative dma_in_range() implementation

["Michał Pecio" <michal.pecio@gmail.com>]

Here's my attempt at it, the patch goes on top of the whole series. It
compiles and passes basic testing (some URBs completed, some unlinked,
no errors or other malfunction apparent).

The idea is to translate dma to trb* ASAP and work with that, because
all data structures use trb* so the code gets shorter and less verbose.
This approach is also easy to adapt to changes in handle_tx_event(),
for example it could trivially return trb* instead of seg* or take trb*
(with or without accompanying seg*) and return bool or even seg*.

I tried to make the common case (start_seg == end_seg) fast, both for
hit and miss. 5 comparisons if the range wraps around, 4 if it doesn't.

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 600842425f6d..b18e7fd7d01e 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -300,42 +300,36 @@ static struct xhci_segment *dma_in_range(struct xhci_segment *start_seg, union x
 					 dma_addr_t dma)
 {
 	struct xhci_segment *seg = start_seg;
+	union xhci_trb *trb;
 
-	if (start_seg == end_seg) {
-		if (start_trb <= end_trb) {
-			if (xhci_trb_virt_to_dma(start_seg, start_trb) <= dma &&
-			    dma <= xhci_trb_virt_to_dma(end_seg, end_trb))
-				return seg;
-			return NULL;
+	/* check if dma is a TRB in start_seg */
+	if (in_range(dma, seg->dma, TRB_SEGMENT_SIZE)) {
+		trb = seg->trbs + (dma - seg->dma) / sizeof(*trb);
+
+		if (trb >= start_trb)
+			/* check if the range covers it and we are done */
+			return (end_seg != seg || trb <= end_trb) ? seg : NULL;
+
+		/* check if the range circles back to the beginning of start_seg */
+		return (end_seg == seg && end_trb < start_trb && trb <= end_trb) ? seg : NULL;
+	}
+
+	/* stop if the range doesn't pass through any other segment */
+	if (end_seg == seg && (end_trb >= start_trb || seg->next == seg))
+		return NULL;
+
+	/* search remaining segments knowing that start_trb isn't there */
+	do {
+		seg = seg->next;
+
+		if (in_range(dma, seg->dma, TRB_SEGMENT_SIZE)) {
+			trb = seg->trbs + (dma - seg->dma) / sizeof(*trb);
+
+			return (seg != end_seg || trb <= end_trb) ? seg : NULL;
 		}
+	} while (seg != end_seg && seg != start_seg);
 
-		/* Edge case, the TD wrapped around to the start segment. */
-		if (xhci_trb_virt_to_dma(end_seg, end_trb) < dma &&
-		    dma < xhci_trb_virt_to_dma(start_seg, start_trb))
-			return NULL;
-		if (seg->dma <= dma && dma <= (seg->dma + TRB_SEGMENT_SIZE))
-			return seg;
-		seg = seg->next;
-	}
-
-	/* Loop through segment which don't contain the DMA address. */
-	while (dma < seg->dma || (seg->dma + TRB_SEGMENT_SIZE) <= dma) {
-		if (seg == end_seg)
-			return NULL;
-
-		seg = seg->next;
-		if (seg == start_seg)
-			return NULL;
-	}
-
-	if (seg == start_seg) {
-		if (dma < xhci_trb_virt_to_dma(start_seg, start_trb))
-			return NULL;
-	} else if (seg == end_seg) {
-		if (xhci_trb_virt_to_dma(end_seg, end_trb) < dma)
-			return NULL;
-	}
-	return seg;
+	return NULL;
 }
 
 static struct xhci_segment *trb_in_td(struct xhci_td *td, dma_addr_t dma)