Re: correctly handling EPROTO

[Thinh Nguyen <Thinh.Nguyen@synopsys.com>]

On Thu, Mar 19, 2026, Mathias Nyman wrote:
> On 3/19/26 03:56, Alan Stern wrote:
> 
> > Just to be clear, are you saying there's no way for an xHC to restart a
> > (host-side) halted non-SuperSpeed endpoint without setting the toggle
> > back to 0?
> 
> There is.
> A reset endpoint command with a TSP flag (transfer state preserve)
> clear the host side halt and preserve the toggle state.
> 
> It's used for soft-retry purposes, retrying a transfer after
> a transaction error. This is also the only use-case described in xHCI specification.

When the xhci gives back the URB with -EPROTO, that's when it gives up
soft-retrying. The TRBs corresponding to the URB are done. Once the
URB is given back, sending new ones should not revive or continue the
previous TRBs.

> 
> Unclear what happens if we clear the host side halt, preserving the toggle, and
> then ask host to move to the next URB

For bulk, if we're trying to preserve the toggle, we can't move on
unless the soft retry succeed and the transfer completed right?

BR,
Thinh

> 
> Could be worth giving people a way to try it out somehow.
> Maybe option to enable it via debugfs, maybe a quirk, or event just provide a patch.
> See how different xHC hosts behave
> 
> -Mathias
> 
>