Re: f_mass_storage vs drivers/target

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

On Thu, 22 Aug 2019, Benjamin Herrenschmidt wrote:

> On Thu, 2019-08-22 at 14:58 +1000, Benjamin Herrenschmidt wrote:
> > 
> > Ah lovely ... the 338x fails in EP autoconf with f_tcm, digging...
> > 
> > While digging I found this gem:
> > 
> > 	/* USB3380: use same address for usb and hardware endpoints */
> > 	snprintf(name, sizeof(name), "ep%d%s", usb_endpoint_num(desc),
> > 			usb_endpoint_dir_in(desc) ? "in" : "out");
> > 	ep = gadget_find_ep_by_name(_gadget, name);
> > 	if (ep && usb_gadget_ep_match_desc(_gadget, ep, desc, ep_comp))
> > 		return ep;
> > 
> > Any idea what's that supposed to achieve ?

It looks like in one mode, the endpoint number has to be the value 
predetermined by the hardware.  In the other mode, any hardware 
endpoint can be assigned any endpoint number.

> > When ep_match is called, usb_endpoint_num() hasn't been set yet so
> > it's always 0 and always fails... or am I missing something ?
> 
> Two problems:
> 
>  - net2280.c doesn't set a max EP size, so autoconfig fails since
> f_tcm specifies one. What about this ?
> 
> --- a/drivers/usb/gadget/udc/core.c
> +++ b/drivers/usb/gadget/udc/core.c
> @@ -940,12 +940,14 @@ int usb_gadget_ep_match_desc(struct usb_gadget *gadget,
>         if (usb_endpoint_dir_out(desc) && !ep->caps.dir_out)
>                 return 0;
>  
> -       if (max > ep->maxpacket_limit)
> +       if (ep->maxpacket_limit && max > ep->maxpacket_limit)
>                 return 0;
> 
> (ie assume that ep->maxpacket_limit 0 means the UDC supports any
> legal size)

That looks reasonable.

>  - No UDC driver other than dummy sets max_streams, and f_tcm requires 4,
> so f_tcm will fail with *any* superspeed UDC driver as far as I can tell.
> 
> Was it ever tested with USB 3 ?

Note that USB 2 does not support streams at all.

> I'm not sure what the right fix here yet is as I yet have to learn about
> what those USB3 streams are :-) For now I've commented things out.

They are for multiplexing multiple data streams over a single USB 
endpoint.  As far as I know, the only use case for such a thing is USB 
Mass Storage.

Alan Stern

> It's still not working yet as configuring f_tcm seems to be a black art
> with no useful documentation or examples anywhere (the device shows up on
> the host but doesn't bind to any mass storage driver ... yet).
> 
> Cheers,
> Ben.