Re: [PATCH] xhci: fix lost bounce buffers on TDs spanning several ring segments

[Michal Pecio <michal.pecio@gmail.com>]

On Tue, 11 Aug 2026 10:42:37 +0200, Michal Pecio wrote:
> The code looks correct, though I would do it differently: store the
> last bounce_seg and run the loop from td->start_seg to td->bounce_seg.
> This avoids adding the 'if' during enqueue, and still works correctly
> if the TD wraps around the whole ring so that end_seg == start_seg.
> 
> The driver is never supposed to create such TDs (they break the ring
> expansion procedure) but I prefer more robust code if it costs nothing.
> Bugs happen, or expansion could theoretically become more flexible.
> 
> Regards,
> Michal
> 
> > +	seg = td->bounce_seg;
> > +	for (i = 0; i < ring->num_segs; i++) {
> > +		if (seg->bounce_len)
> > +			xhci_unmap_one_bounce_buffer(xhci, ring, td, seg);
> > +		if (seg == td->end_seg)
> > +			break;
> > +		seg = seg->next;
> > +	}

Oops, sorry, Mathias is right, this loop may incorrectly unmap a buffer
belonging to a later TD which starts somewhere in td->end_seg.

I think the alternative procedure I suggested is free of this problem.

- if td has bounce_seg, then it surely spans beyond start_seg, so we
  can safely unmap start_seg's bounce buffer
- if start_seg == bounce_seg then this was the last bounce to unmap,
  because it's impossible for end_trb to also be a bonuce.
- otherwise, we can continue to blindly unmap until reachng bounce_seg
- after unmapping bounce_seg, there is nothing more left to unmap

Regards,
Michal