Re: [PATCH 0/1] usb: xhci: Queue URB_ZERO_PACKET as one TD

[Mathias Nyman <mathias.nyman@linux.intel.com>]

On 9.9.2025 20.38, Michal Pecio wrote:
> On Tue, 9 Sep 2025 16:04:33 +0300, Mathias Nyman wrote:
>> Adding the zero-length TRB to the original TD when we need to send a
>> zero-length packet would simplify things, and I would otherwise fully
>> support this, but the xHCI spec is pretty clear that it requires a
>> dedicated TD for zero-length transactions.
> 
> You are right of course, an empty TRB in a TD would simply send no
> data, or maybe it's a TRB Error, I'm not sure.
> 
> But this is not what this patch is about - the trick is to use an
> *unchained* TRB, which is a separate TD from HW's perspective, and
> to count it as part of the same TD from the driver's perspective.

Ok, I see.
The whole TD without completion flag does worry me a bit.

We need to make sure stop/stald mid TD cases work, and  urb length is
set correctly.

> 
> Control URBs are like that and they work fine. They can halt on any
> TRB (which are all unchained, per spec) and the whole URB goes out.
> 
> This bug is (probably?) low impact, but it bothers me because it's
> a design flaw: either non-isoc multi-TD URBs are supported, or they
> are not. One or another part of the driver needs to adapt.

There is a risk that this is one of those "cure is worse than the
disease" cases.

Thanks
Mathias