Re: Pass transfer_buffer to gadget drivers

[Andrey Konovalov <andreyknvl@google.com>]

On Fri, Jun 7, 2019 at 2:43 PM Felipe Balbi
<felipe.balbi@linux.intel.com> wrote:
>
>
> Hi,
>
> Andrey Konovalov <andreyknvl@google.com> writes:
> >> >> Andrey Konovalov <andreyknvl@google.com> writes:
> >> >> > I've noticed that when the host performs a control request,
> >> >> > urb->transfer_buffer/transfer_buffer_length are not passed to the
> >> >> > gadget drivers via the setup() call, the only thing that is passed is
> >> >> > the usb_ctrlrequest struct. Is there a way to get the transfer_buffer
> >> >> > from within a gadget driver? If not, what approach would the best to
> >> >> > implement this?
> >> >>
> >> >> I think you need to further explain what you mean here.
> >> >>
> >> >> What do you mean by gadget driver in this case?
> >> >>
> >> >> If you mean the drivers under drivers/usb/gadget/{function,legacy}
> >> >> directories then there's no way that they can have access to anything
> >> >> from the host.
> >> >>
> >> >> Remember that gadget and host are two completely distinct units. The
> >> >> only thing they share is a USB cable. When it comes to Control
> >> >> Transfers, if a data stage is necessary, that must be encoded in the
> >> >> wLength field of the control structure.
> >> >>
> >> >> Also, host side does *not* pass its usb_ctrlrequest struct to the
> >> >> gadget, it passes a series of 8 bytes which are oblivious to where in
> >> >> memory they were from the host point of view.
> >> >>
> >> >> If if you have the same machine acting as both host and device, each
> >> >> side has no knowledge of that fact.
> >> >
> >> > Hi Felipe,
> >> >
> >> > What I meant is that any module (gadget driver) that implements
> >> > usb_gadget_driver struct callbacks and registers it, will only get
> >> > usb_ctrlrequest through the setup() callback, but not the
> >> > transfer_buffer/length.
> >>
> >> A control request is *always* 8 bytes. That's mandated by the USB
> >> specification.
> >>
> >> > And therefore it can't access the data that is
> >> > attached to a control request.
> >>
> >> There is no data attached to a control request. A Control Transfer is
> >> composed of 2 or 3 stages:
> >>
> >> - SETUP stage
> >>         an 8 byte transfer descriptor type thing
> >>
> >> - (optional) Data stage
> >>         if wLength of control request contains a value > 0, then this
> >>         stage fires up to transfer the amount of data communicated in
> >>         wLength (during previous stage).
> >>
> >> - Status Stage
> >>         A zero length transfer to communicate successful end of transfer
> >>         (in case it completes fine) or an error (in case of STALL
> >>         condition).
> >
> > Hm, then why does the usb_control_msg() function accepts a data and
> > size arguments? Which are described in the comment as "pointer to the
> > data to send" and "length in bytes of the data to send" accordingly?
> > Or is this the buffer for the response?
>
> That's for the data stage :-)

Sure =)

>
> usb_control_msg() is an upper lever API to encode and entire Control
> Transfer (all stages of it).
>
> What is the problem you see, then?

The problem is that I want to receive that data (from the data stage)
from within my gadget driver module. But it's not passed to the
setup() callback. And the question is: how do I do that then?

>
> --
> balbi