---
** [tickets:#118] jlink.c: Bug in jlink_usb_io()**
**Status:** new
**Milestone:** 0.9.0
**Created:** Thu Feb 04, 2016 10:33 AM UTC by Alex Gruener, CTO SEGGER
**Last Updated:** Thu Feb 04, 2016 10:33 AM UTC
**Owner:** nobody
Hi,
I am writing here as an official SEGGER J-Link employee.
There is a bug in jlink_usb_io() in jlink.c that comes up with J-Link models
that incorporate USB Hi-Speed.
It does not occur with older USB Full-Speed models:
~~~~
static int jlink_usb_io(struct jlink *jlink, int out_length, int in_length)
{
[...]
/*
* Section 4.2.4 IN-transaction:
* read dummy 0-byte packet if transaction size is
* multiple of 64 bytes but not max. size of 0x8000
*/
if ((in_length % 64) == 0 && in_length != 0x8000) {
char dummy_buffer;
result = usb_bulk_read_ex(jlink->usb_handle, jlink_read_ep,
&dummy_buffer, 1, JLINK_USB_TIMEOUT);
if (result != 0) {
LOG_ERROR("dummy byte read failed");
return ERROR_JTAG_DEVICE_ERROR;
}
}
return ERROR_OK;
}
~~~~
A dummy byte request should only be done if:
in_length == multiple of MaxPacketSize and not a multiple of 2 KB
MaxPacketSize for Full-Speed: 64 bytes
MaxPacketSize for Hi-Speed: 512 bytes
Correct pseudo-code:
~~~~
MaxPacketSize = _GetEPMaxPacketSize(iEP);
if ((in_length % MaxPacketSize) == 0) {
if ((in_length % 0x800) != 0) { // Needed for compatibility with J-Link
Windows driver behavior
_ReadDummyByte();
}
}
~~~~
In case if additional information is needed etc., please to do not hesitate to
contact me or SEGGER directly.
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