Re: dwc3: unusual handling of setup requests with wLength == 0

[Alan Stern]

On Fri, Aug 25, 2023 at 03:36:36AM +0200, Andrey Konovalov wrote:
> So to summarize the issue wrt the 0-length requests:
> 
> 1. UDC drivers must always delay the status stage for 0-length
> requests until the gadget driver queues an empty request.

Any request, not necessarily empty (although it should be).  The request 
queued by the gadget driver _is_ the Status stage response; in this 
situation the UDC driver doesn't create one automatically.

> 2. Many UDC drivers do not do this, and only delay the status stage
> when USB_GADGET_DELAYED_STATUS is returned from ->setup(). Some
> drivers also assume that only a SET_CONFIGURATION request can be
> delayed.
> 
> 3. All such UDC drivers should be fixed and USB_GADGET_DELAYED_STATUS
> should be contained within the composite framework.
> 
> 4. Fixing all such UDC drivers is a non-trivial amount of work, but
> this is the goal to strive towards.
> 
> Alan, would it be acceptable if I add custom handling of
> USB_GADGET_DELAYED_STATUS to Raw Gadget in the meantime? It would be

Sure.  It's your driver; do whatever you want with it.  :-)

> great to keep it at least somewhat working with dwc3. I can also do it
> for GadgetFS, if you think it's a good idea.

I suspect gadgetfs doesn't need it.  But go ahead and look through the 
code to check for yourself; I might be wrong.

> I can also add some clarifying comments for USB_GADGET_DELAYED_STATUS
> and ->setup() to hopefully avoid having new UDC drivers being added
> with the same issue (e.g. cdns2 and renesas_usbf with the same issue
> were added just recently).

Good idea, although mistakes like this tend to propagate more through 
copy-and-paste than by failures of documentation.

> Maybe it's also a good idea to add a checkpatch.pl check for using
> USB_GADGET_DELAYED_STATUS in UDC drivers. Or maybe ask Greg to keep an
> eye out for this?

I wouldn't try to modify checkpatch.pl.

Alan Stern