Hello Dave,
On Monday, 2. November 2009 05:29:56 David Brownell wrote:
> When running an FT2232H at full speed, we saw a new failure mode that
> turned out to be a bug in libftdi. Current git:
>
> int ftdi_read_data(struct ftdi_context *ftdi, unsigned char *buf, int
> size) {
> int offset = 0, ret = 1, i, num_of_chunks, chunk_remains;
> int packet_size;
>
> // New hi-speed devices from FTDI use a packet size of 512 bytes
> if (ftdi->type == TYPE_2232H || ftdi->type == TYPE_4232H)
> packet_size = 512;
> else
> packet_size = 64;
>
> ....
>
> So there's a clear and *INCORRECT* assumption that highspeed-capable
> devices are never going to run at full speed, where packet_size != 512
> but instead is the traditional 64 bytes (on these chips).
>
> Failure mode: two-byte "modem" status codes bubbling up in the middle
> of JTAG transactions. Rude...
>
> I have no patch for this; just reporting the bug. It's clearly fixable
> on Linux, which is our priority (OpenOCD, for JTAG) by asking sysfs for
> the device's "speed" attribute (12 or 480 MHz), or (more awkwardly) the
> endpoint's wMaxPacketSize. Other operating systems -- no comments.
Thanks for reporting this issue. Maybe there's a way to ask libusb
about the maximum packet size, I'll have to check that. Otherwise
we would need to go the sysfs route.
Does anyone know if it's a valid thing to do for a USB device
to switch to a lower speed during one "session"?
Cheers,
Thomas
--
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