Re: [PATCH 0/4] usb: usbtmc: fix undersized high-speed bulk packets on xHCI

[Nicolai Buchwitz <nb@tipi-net.de>]

Hi Michal

On 20.10.2026 18:42, Michal Pecio wrote:
> [...]

> But SDM4065A is apparently pretty modern (2025) gear marketed for
> "automatic production lines", so it's odd that it has such issues.
> Has anyone looked if the same out-of-spec descriptor shows up on
> Windows or (don't laught at my optimism) consulted vendor support?

Not yet, but I'll check on Windows. The vendor(s) don't really seem to
care about the bug (surprise!). It is known for quite a while and even
not fixed with the latest 2026 firmware for the siglent. This makes me
assume that either most of the professional users a) use Windows or b)
use the lan interface (which we do for most of the stations).

> 
> Maybe Windows xHCI driver tries to enable out-of-spec maxpacket?
> We could do that too, though IIRC not all controllers are happy,
> so a SW workaround may be unavoidable.

Did some quick tests and it looks good so far. If the usb maintainers
can agree on making it less restrictive (/ compliant) it would save
lots of efforts and the quirk would be down to the ZLP patch. In my
tests all three devices worked with

diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
index 8a8636279e844..c4306482874eb 100644
--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -1505,7 +1505,9 @@ int xhci_endpoint_init(struct xhci_hcd *xhci,
                 err_count = 3;
         /* HS bulk max packet should be 512, FS bulk supports 8, 16, 32 
or 64 */
         if (usb_endpoint_xfer_bulk(&ep->desc)) {
-               if (udev->speed == USB_SPEED_HIGH)
+               if (udev->speed == USB_SPEED_HIGH && max_packet != 512)
+                       xhci_warn(xhci, "EXPERIMENT: keeping HS bulk 
maxp %u\n", max_packet);
+               else if (udev->speed == USB_SPEED_HIGH)
                         max_packet = 512;
                 if (udev->speed == USB_SPEED_FULL) {
                         max_packet = rounddown_pow_of_two(max_packet);

So this looks like the better fix. How would we want it gated, a USB
core quirk flag per device, or per controller?

> [...]

>> The Rigols still have one issue I can't work around. They split
>> replies over 500 bytes into several transfers, and a follow-on
>> transfer that ends exactly on a packet boundary never arrives. Raw
>> libusb shows the same, so I guess it's another firmware bug. Should
>> be fine for most use cases though.
> 
> What exactly is the sequence of URBs you see vs would want to see?

The Rigols split anything over 500 bytes into two transfers:

   REQUEST_DEV_DEP_MSG_IN #1
   IN: 8 x 64 bytes (12 byte header + 500 data), then a ZLP
   REQUEST_DEV_DEP_MSG_IN #2
   IN: header + the rest

That works fine, unless the second transfer happens to end exactly on
a 64 byte boundary (e.g. 640 bytes). Then the device just never answers
request #2 and the first IN times out. I also tried leaving the ZLP
from #1 pending until after request #2. That fixes the boundary case,
but then the device randomly ignores request #2 instead (roughly 1 in
5). Raw libusb behaves the same, also with maxpacket 64 in xHCI, so I'm
pretty sure it's just another firmware bug. What I'd want to see is
simply the second transfer arriving :)

> [...]

Thanks,
Nicolai