[PATCH 4/8] selftests/vfio: igb: Extend memcpy completion timeout for line-rate hardware

[Alex Williamson]

The submitted driver waits at most 1 ms (100 * 10 us) for the last RX
descriptor to be written back.  QEMU's emulated loopback is synchronous:
by the time igb_memcpy_wait() runs, the receive descriptors are already
written back.  Real 82576 hardware processes the descriptor ring at
line rate.

max_memcpy_size is (RING_SIZE - 1) * IGB_MAX_CHUNK_SIZE, approximately
4 MB, split into 4095 1 KB frames.  At 1 Gb/s line rate (~125 MB/s),
4 MB takes ~32 ms on the wire, plus per-frame preamble, SFD,
inter-frame gap and FCS overhead (~3%) and descriptor fetch/writeback
latency.  The 1 ms cap times out long before any valid transfer can
complete.

Wait up to ~200 ms (200 iterations * 1 ms) for descriptor writeback
before returning -ETIMEDOUT.  ~6x the line-rate floor leaves comfortable
headroom for host scheduling jitter and slower PCIe paths, while still
bounding the intentional invalid-DMA tests (mix_and_match) so they
recover quickly on real faults.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
---
 .../testing/selftests/vfio/lib/drivers/igb/igb.c  | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/vfio/lib/drivers/igb/igb.c b/tools/testing/selftests/vfio/lib/drivers/igb/igb.c
index d44a08a36171..2297382d7c26 100644
--- a/tools/testing/selftests/vfio/lib/drivers/igb/igb.c
+++ b/tools/testing/selftests/vfio/lib/drivers/igb/igb.c
@@ -422,11 +422,22 @@ static int igb_memcpy_wait(struct vfio_pci_device *device)
 	prev_tail = (igb->rx_tail + RING_SIZE - 1) % RING_SIZE;
 	rx = &igb->rx_ring[prev_tail];
 
-	retries = 100;
+	/*
+	 * Real 82576 hardware processes the descriptor ring at line rate.
+	 * max_memcpy_size = (RING_SIZE - 1) * IGB_MAX_CHUNK_SIZE ~= 4 MB,
+	 * split into 4095 1 KB frames.  At 1 Gb/s (~125 MB/s) the worst
+	 * valid memcpy takes ~32 ms on the wire, plus per-frame preamble,
+	 * SFD, IFG and FCS overhead (~3%) and descriptor fetch/writeback
+	 * latency.  Wait up to ~200 ms before declaring the device hung;
+	 * ~6x the line-rate floor leaves comfortable headroom for host
+	 * scheduling jitter while keeping the intentional invalid-DMA
+	 * tests bounded.
+	 */
+	retries = 200;
 	while (retries-- > 0) {
 		if (rx->wb.status_error & 1)
 			break;
-		usleep(10);
+		usleep(1000);
 	}
 
 	igb_irq_clear(igb);
-- 
2.51.0