git: 1d445ae68446 - main - dpaa/eth: fast-path single-page TX frame build

The branch main has been updated by jhibbits:

URL: https://cgit.FreeBSD.org/src/commit/?id=1d445ae68446771c78161a3eca35acdb04ef78ab

commit 1d445ae68446771c78161a3eca35acdb04ef78ab
Author:     Justin Hibbits <jhibbits@FreeBSD.org>
AuthorDate: 2026-08-10 18:24:08 +0000
Commit:     Justin Hibbits <jhibbits@FreeBSD.org>
CommitDate: 2026-08-22 16:03:31 +0000

    dpaa/eth: fast-path single-page TX frame build
    
    The TX SG-build loop in dpaa_eth_if_start_locked() walked page
    boundaries with PAGE_MASK arithmetic even for buffers that lived
    entirely within one page -- the common case, since MCLBYTES
    is smaller than PAGE_SIZE.  Add a fast path that emits a single SGT
    entry for wholly-in-one-page segments and skips the inner while
    entirely.
    
    Fix the following bugs while we're here:
    
    1. "if (m->m_len == 0) continue;" in the outer loop never
       advanced m -- any zero-length mbuf hung the TX path in an
       infinite loop.  Fix this by switching to a for loop, with the
       advancement in the post-clause.
    
    2. In the inner (page-splitting) loop, the cap
       "if (m->m_len < ssize) ssize = m->m_len;" compared against
       the mbuf's original length, not the remaining bytes.  A single mbuf
       whose data started mid-page and ran into a second page would produce
       a second SGT entry with ssize > rem, over-reading past the buffer end
       into whatever followed in kernel memory.  Fixed by tracking a local
       rem and capping ssize against it.
    
    3. If the whole mbuf chain consisted of zero-length segments,
       the final-flag store "fi_sgt[i - 1].final = 1" wrote to
       index -1.  Reject empty frames up front now instead.
    
    As part of this, rename the inner counter from dsize to rem for clarity
    instead of playing double-duty in both inner and outer loops.
---
 sys/dev/dpaa/dpaa_eth.c | 57 ++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 42 insertions(+), 15 deletions(-)

diff --git a/sys/dev/dpaa/dpaa_eth.c b/sys/dev/dpaa/dpaa_eth.c
index 2fb902c4d300..e2826eb70b14 100644
--- a/sys/dev/dpaa/dpaa_eth.c
+++ b/sys/dev/dpaa/dpaa_eth.c
@@ -670,7 +670,7 @@ dpaa_eth_tx_add_csum(struct dpaa_eth_frame_info *fi)
 void
 dpaa_eth_if_start_locked(struct dpaa_eth_softc *sc)
 {
-	vm_size_t dsize, psize, ssize;
+	vm_size_t psize, ssize;
 	struct dpaa_eth_frame_info *fi;
 	unsigned int i;
 	struct mbuf *m0, *m;
@@ -710,44 +710,71 @@ dpaa_eth_if_start_locked(struct dpaa_eth_softc *sc)
 		}
 
 		i = 0;
-		m = m0;
 		psize = 0;
-		dsize = 0;
 		fi->fi_mbuf = m0;
 
-		while (m && i < DPAA_NUM_OF_SG_TABLE_ENTRY) {
+		for (m = m0; m != NULL && i < DPAA_NUM_OF_SG_TABLE_ENTRY;
+		    m = m->m_next) {
+			vm_size_t rem;
+
 			if (m->m_len == 0)
 				continue;
 
-			dsize = m->m_len;
 			vaddr = (vm_offset_t)m->m_data;
-			while (dsize > 0 && i < DPAA_NUM_OF_SG_TABLE_ENTRY) {
+			rem = m->m_len;
+
+			/*
+			 * Fast path: the whole segment lives inside one
+			 * page.  Covers every default-cluster mbuf
+			 * (MCLBYTES < PAGE_SIZE) and skips the split loop
+			 * in the common case.
+			 */
+			if ((vaddr & PAGE_MASK) + rem <= PAGE_SIZE) {
+				fi->fi_sgt[i].addr = dpaa_eth_va_to_phys(vaddr);
+				fi->fi_sgt[i].length = rem;
+				fi->fi_sgt[i].extension = 0;
+				fi->fi_sgt[i].final = 0;
+				fi->fi_sgt[i].bpid = 0;
+				fi->fi_sgt[i].offset = 0;
+				psize += rem;
+				i++;
+				continue;
+			}
+
+			/*
+			 * Slow path: mbuf crosses at least one page
+			 * boundary (jumbo cluster, or an unusually-offset
+			 * external buffer).  Emit one SGT entry per
+			 * contiguous physical span.
+			 */
+			while (rem > 0 && i < DPAA_NUM_OF_SG_TABLE_ENTRY) {
 				ssize = PAGE_SIZE - (vaddr & PAGE_MASK);
-				if (m->m_len < ssize)
-					ssize = m->m_len;
+				if (rem < ssize)
+					ssize = rem;
 
 				fi->fi_sgt[i].addr = dpaa_eth_va_to_phys(vaddr);
 				fi->fi_sgt[i].length = ssize;
-
 				fi->fi_sgt[i].extension = 0;
 				fi->fi_sgt[i].final = 0;
 				fi->fi_sgt[i].bpid = 0;
 				fi->fi_sgt[i].offset = 0;
 
-				dsize -= ssize;
+				rem -= ssize;
 				vaddr += ssize;
 				psize += ssize;
 				i++;
 			}
 
-			if (dsize > 0)
+			if (rem > 0)		/* SGT full mid-mbuf */
 				break;
-
-			m = m->m_next;
 		}
 
-		/* Check if SG table was constructed properly */
-		if (m != NULL || dsize != 0) {
+		/*
+		 * Reject the frame if we didn't consume the whole chain
+		 * (SGT full mid-frame) or if the chain produced no SGT
+		 * entries at all (all-zero-length mbufs).
+		 */
+		if (m != NULL || i == 0) {
 			dpaa_eth_fi_free(sc, fi);
 			m_freem(m0);
 			continue;