firehose/usb: Explicitly handle ZLP on USB read transfers

The assumption that every other Firehose read transfer is null is
incorrect. A Zero-Length Packet (ZLP) is only sent when the received
transfer size is a multiple of the USB IN endpoint's Max Packet Size.
This is a way for the device to indicated end-of-transfer.

Unconditionally expecting an empty packet was not an issue with USB 2.0,
where the typical 512-byte sector size matches the bulk IN endpoint's
Max Packet Size. However, with USB SuperSpeed (Max Packet Size = 1024),
attempting to read an empty packet after a 512/1024-byte transfer is
unnecessary and fails (timeout), resulting in errors during storage
device sector size discovery:
```
waiting for programmer...
qdl: failed to read: Resource temporarily unavailable
```

Fix and move ZLP handling to bus-specific code (usb.c).

Reported-by: Danilo Leo <[email protected]>
Signed-off-by: Loic Poulain <[email protected]>

Author: Loic Poulain

firehose.c

@@ -642,7 +642,6 @@ static int firehose_issue_read(struct qdl_device *qdl, struct read_op *read_op,
 	size_t left;
 	int ret;
 	int n;
-	bool expect_empty;
 
 	buf = malloc(qdl->max_payload_size);
 	if (!buf)
@@ -681,22 +680,15 @@ static int firehose_issue_read(struct qdl_device *qdl, struct read_op *read_op,
 	t0 = time(NULL);
 
 	left = read_op->num_sectors;
-	expect_empty = false;
-	while (left > 0 || expect_empty) {
+	while (left > 0) {
 		chunk_size = MIN(qdl->max_payload_size / sector_size, left);
 
 		n = qdl_read(qdl, buf, chunk_size * sector_size, 30000);
 		if (n < 0) {
 			err(1, "failed to read");
 		}
 
-		if (n == 0 && expect_empty) {
-			// Every second transfer is empty during this stage.
-			expect_empty = false;
-			continue;
-		} else if (expect_empty) {
-			err(1, "expected empty transfer but received non-empty transfer during read");
-		} else if ((size_t)n != chunk_size * sector_size) {
+		if ((size_t)n != chunk_size * sector_size) {
 			err(1, "failed to read full sector");
 		}
 
@@ -715,10 +707,6 @@ static int firehose_issue_read(struct qdl_device *qdl, struct read_op *read_op,
 		}
 
 		left -= chunk_size;
-#ifndef _WIN32
-		// on mac/linux, every other response is empty
-		expect_empty = true;
-#endif
 
 		if (!quiet)
 			ux_progress("%s", read_op->num_sectors - left, read_op->num_sectors, read_op->filename);

usb.c

@@ -244,6 +244,14 @@ static int usb_read(struct qdl_device *qdl, void *buf, size_t len, unsigned int
 	if (ret == LIBUSB_ERROR_TIMEOUT && actual == 0)
 		return -ETIMEDOUT;
 
+	/* If what we read equals the endpoint's Max Packet Size, consume the ZLP explicitly */
+	if ((len == actual) && !(actual % qdl_usb->in_maxpktsize)) {
+		ret = libusb_bulk_transfer(qdl_usb->usb_handle, qdl_usb->in_ep,
+					   NULL, 0, NULL, timeout);
+		if (ret)
+			warnx("Unable to read ZLP: %s", libusb_strerror(ret));
+	}
+
 	return actual;
 }