(+CC roger as the author of the USB_GADGET_DELAYED_STATUS mechanism)
On 26/01/2023 23:57, Thinh Nguyen wrote:
On Thu, Jan 26, 2023, Alan Stern wrote:
On Thu, Jan 26, 2023 at 07:31:34PM +0000, Thinh Nguyen wrote:
On Thu, Jan 26, 2023, Dan Scally wrote:
Hi Thinh
On 26/01/2023 00:20, Thinh Nguyen wrote:
On Tue, Jan 24, 2023, Dan Scally wrote:
Hi Thinh
I'm trying to update the DWC3 driver to allow the status phase of a
transaction to be explicit; meaning the gadget driver has the choice to
either Ack or Stall _after_ the data phase so that the contents of the data
phase can be validated. I thought that I should be able to achieve this by
preventing dwc3_ep0_xfernotready() from calling dwc3_ep0_do_control_status()
(relying on an "explicit_status" flag added to the usb_request to decide
whether or not to do so) and then calling it manually later once the data
phase was validated by the gadget driver (or indeed userspace). A very
barebones version of my attempt to do that looks like this:
We shouldn't do this. At the protocol level, there must be better ways
to do handshake than relying on protocol STALL _after_ the data stage.
Note that not all controllers support this.
Maybe I'm misunderstanding, but isn't this how the USB spec expects it to
work? Reading "Reporting Status Results (8.5.3.1)" in the USB 2.0 spec for
the status stage in a control write it says "The function responds with
either a handshake or a zero-length data packet to indicate its current
status", and the handshake can be either STALL or NAK. If we can't do this,
how else can we indicate to the host that the data sent during a control out
transfer is in some way invalid?
My concern is from the documentation note[*] added from this commit:
579c2b46f74 ("USB Gadget: documentation update")
When the gadget subsystem was originally designed, it made no allowance
for sending a STALL in the status stage. The UDC drivers existing at
that time would automatically send their own zero-length status packet
when the control data was received.
Drivers written since then have copied that approach. They had to, if
they wanted to work with the existing gadget drivers. So the end result
is that fully supporting status stalls will require changing pretty much
every UDC driver.
As for whether the UDC hardware has support... I don't know. Some of
the earlier devices might not, but I expect that the more popular recent
designs would provide a way to do it.
Right, it's just a bit concerning when the document also noted this:
"Note that some USB device controllers disallow protocol stall responses
in some cases."
It could be just for older controllers as you mentioned.
Hi Dan,
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@xxxxxxxxxxxxxxxx/
Regarding this question:
How else can we indicate to the host that the data sent during a
control out transfer is in some way invalid?
Typically there should be another request checking for the command
status. I suppose if we use protocol STALL, you only need to send status
request check on error cases.
Thanks,
Thinh
