Re: Explicit status phase for DWC3

[Alan Stern <stern@rowland.harvard.edu>]

On Thu, Feb 02, 2023 at 10:12:45AM +0000, Dan Scally wrote:
> (+CC roger as the author of the USB_GADGET_DELAYED_STATUS mechanism)
> 
> On 26/01/2023 23:57, Thinh Nguyen wrote:
> > We should already have this mechanism in place to do protocol STALL.
> > Please look into delayed_status and set halt.
> 
> 
> Thanks; I tried this by returning USB_GADGET_DELAYED_STATUS from the
> function's .setup() callback and later (after userspace checks the data
> packet) either calling usb_ep_queue() or usb_ep_set_halt() and it does seem
> to be working. This surprises me, as my understanding was that the purpose
> of USB_GADGET_DELAYED_STATUS  is to pause all control transfers including
> the data phase to give the function driver enough time to queue a request
> (and possibly only for specific requests). Regardless though I think the
> conclusion from previous discussions on this topic (see [1] for example) was
> that we don't want to rely on USB_GADGET_DELAYED_STATUS to do this which is
> why I had avoided it in the first place. A colleague made a series [2] some
> time ago that adds a flag to usb_request which function drivers can set when
> queuing the data phase request. UDC drivers then read that flag to decide
> whether to delay the status phase until after another usb_ep_queue(), and
> that's what I'm trying to implement here.
> 
> 
> [1] https://lkml.org/lkml/2018/10/10/138
> 
> [2] https://patchwork.kernel.org/project/linux-usb/patch/20190124030228.19840-5-paul.elder@ideasonboard.com/

I'm in favor of the explicit_status approach from [2].  In fact, there 
was a whole series of patches impementing this, and I don't think any of 
them were merged.

Keep in mind that there are two separate issues here:

	Status/data stage for a control-IN or 0-length control-OUT
	transfer.

	Status stage for a non-0-length control-OUT transfer.

The USB_GADGET_DELAYED_STATUS mechanism was meant to help with the 
first, not the second.  explicit_status was meant to help with the 
second; it may be able to help with both.

Alan Stern