usb: Deal with truncated ISO IN transfers

It seems that in some circumstances, an ISO IN transfer can be
truncated by the bus / host.  In such situation we'd currently pass
a non-modulo-32 length to the mux_demux (deframer) code, and it ASSERTs
on that.  Let's try to handle this more gracefully by substituting
random garbage and letting higher layers deal with massive bit errors.

Related: OS#5490
Change-Id: Ic453325b93b0e12727625a1495a948d96df4b542

3 files changed, 29 insertions, 0 deletions

diff --git a/src/e1d.h b/src/e1d.h
index 1c7c4c4..45ecb82 100644
--- a/src/e1d.h
+++ b/src/e1d.h
@@ -59,6 +59,7 @@ enum e1d_line_ctr {
 	LINE_CTR_RX_REMOTE_A,
 	LINE_CTR_FRAMES_MUXED_E1T,
 	LINE_CTR_FRAMES_DEMUXED_E1O,
+	LINE_CTR_USB_ISO_TRUNC,
 };
 
 enum e1d_line_stat_item {
diff --git a/src/intf_line.c b/src/intf_line.c
index 3895653..f41214e 100644
--- a/src/intf_line.c
+++ b/src/intf_line.c
@@ -60,6 +60,7 @@ static const struct rate_ctr_desc line_ctr_description[] = {
 	[LINE_CTR_RX_REMOTE_A] ={ "rx:remote_alarm",		"Rx Frames Reporting Remote Alarm"},
 	[LINE_CTR_FRAMES_MUXED_E1T] = { "tx:frames_muxed",	"E1 Tx Frames multiplexed" },
 	[LINE_CTR_FRAMES_DEMUXED_E1O] = { "rx:frames_demuxed",	"E1 Rx Frames demultiplexed" },
+	[LINE_CTR_USB_ISO_TRUNC] = { "rx:usb_iso_trunc",	"USB ISO packets truncated" },
 };
 
 static const struct rate_ctr_group_desc line_ctrg_desc = {
diff --git a/src/usb.c b/src/usb.c
index 1ff9b43..fcffc12 100644
--- a/src/usb.c
+++ b/src/usb.c
@@ -131,6 +131,33 @@ e1_usb_xfer_in(struct e1_usb_flow *flow, uint8_t *buf, int len, size_t size)
 {
 	if (len == 0)
 		return 0;
+
+	if (len < 4) {
+		LOGPLI(flow->line, DE1D, LOGL_ERROR, "IN EP %02x ISO packet truncated: len = %u\n",
+		       flow->ep, len);
+		line_ctr_add(flow->line, LINE_CTR_USB_ISO_TRUNC, 1);
+		return 0;
+	}
+
+	if (len > 4 && (len - 4) % 32) {
+		/* some ISO IN packet was truncated. Apparently this
+		 * does happen, see https://osmocom.org/issues/5490 -
+		 * there is little we can do here, but instead of the
+		 * earlier ASSERT, we just feed some garbage for the
+		 * last few timeslots, resulting in bit errors etc. */
+		LOGPLI(flow->line, DE1D, LOGL_ERROR, "IN EP %02x ISO packet truncated: len-4 = %u\n",
+		       flow->ep, len - 4);
+		line_ctr_add(flow->line, LINE_CTR_USB_ISO_TRUNC, 1);
+
+		/* The assumption here is that only the last E1 frame is
+		 * truncated, as we have no idea how many E1 frames the
+		 * USB device firmware wanted to send us. */
+		len += 32 - (len % 32);
+		/* don't overflow the underlying buffer */
+		if (len > (int) size)
+			len = size;
+	}
+
 	return e1_line_demux_in(flow->line, buf + 4, len - 4, buf[3] & 0xf);
 }