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libusb: implement zlp flag in libusb transfer

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The USB protocol defines a Zero-Length Packet (ZLP) to signal the end of
a transfer when the data size is an exact multiple of the Maximum Packet
Size (MPS). Without a ZLP in such cases, the device may not be able to
determine that the transfer has completed.

This flag is added to libusb to allow the user send a ZLP in the end
of libusb_xfer.

Reviewed by:    adrian
Sponsored by:   The FreeBSD Foundataion
Differential Revision: https://reviews.freebsd.org/D51759
This commit is contained in:
ShengYi Hung committed 2026-06-22 21:36:48 +08:00
1 parent 964823a18e
commit d84b639a68
2 files changed
+29 -1
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lib/libusb/libusb.h
+1
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@@ -264,6 +264,7 @@ enum libusb_transfer_flags {
LIBUSB_TRANSFER_SHORT_NOT_OK = 1 << 0,
LIBUSB_TRANSFER_FREE_BUFFER = 1 << 1,
LIBUSB_TRANSFER_FREE_TRANSFER = 1 << 2,
LIBUSB_TRANSFER_ADD_ZERO_PACKET = 1 << 3,
};
enum libusb_log_level {
lib/libusb/libusb10.c
+28 -1
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@@ -1419,6 +1419,7 @@ libusb10_bulk_intr_proxy(struct libusb20_transfer *pxfer)
uint32_t actlen;
uint8_t status;
uint8_t flags;
uint8_t tr_flags;
status = libusb20_tr_get_status(pxfer);
sxfer = libusb20_tr_get_priv_sc1(pxfer);
@@ -1471,6 +1472,20 @@ libusb10_bulk_intr_proxy(struct libusb20_transfer *pxfer)
sxfer->curr_data += max_bulk;
sxfer->rem_len -= max_bulk;
/*
* When a zero length packet (ZLP) is requested, ask the
* kernel to terminate the last frame of the transfer with
* a short packet. This appends a ZLP when the data length
* is an exact multiple of the maximum packet size.
*/
tr_flags = libusb20_tr_get_flags(pxfer);
if (sxfer->rem_len == 0 &&
(flags & LIBUSB_TRANSFER_ADD_ZERO_PACKET))
tr_flags |= LIBUSB20_TRANSFER_FORCE_SHORT;
else
tr_flags &= ~LIBUSB20_TRANSFER_FORCE_SHORT;
libusb20_tr_set_flags(pxfer, tr_flags);
libusb20_tr_submit(pxfer);
/* check if we can fork another USB transfer */
@@ -1710,7 +1725,7 @@ libusb_submit_transfer(struct libusb_transfer *uxfer)
struct libusb_super_transfer *sxfer;
struct libusb_device *dev;
uint8_t endpoint;
int err;
int err, mps;
if (uxfer == NULL)
return (LIBUSB_ERROR_INVALID_PARAM);
@@ -1731,6 +1746,18 @@ libusb_submit_transfer(struct libusb_transfer *uxfer)
pxfer0 = libusb10_get_transfer(uxfer->dev_handle, endpoint, 0);
pxfer1 = libusb10_get_transfer(uxfer->dev_handle, endpoint, 1);
mps = libusb_get_max_packet_size(dev, endpoint);
/*
* The ADD_ZERO_PACKET flag only has an effect on host-to-device
* (OUT) transfers whose length is an exact multiple of the maximum
* packet size. Clear it otherwise so the kernel is not asked to
* terminate the transfer with a short packet, see
* libusb10_bulk_intr_proxy().
*/
if ((uxfer->flags & LIBUSB_TRANSFER_ADD_ZERO_PACKET) &&
((endpoint & LIBUSB_ENDPOINT_IN) || uxfer->length % mps != 0))
uxfer->flags &= ~LIBUSB_TRANSFER_ADD_ZERO_PACKET;
if (pxfer0 == NULL || pxfer1 == NULL) {
err = LIBUSB_ERROR_OTHER;