linux-kernel - Re: [PATCH 2/3] usb: gadget: f_midi: free usb request when done

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Date:	Wed, 23 Sep 2015 15:47:56 +0100
From:	Felipe Tonello <eu@...ipetonello.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Peter Chen <peter.chen@...escale.com>,
	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Felipe Balbi <balbi@...com>,
	Andrzej Pietrasiewicz <andrzej.p@...sung.com>
Subject: Re: [PATCH 2/3] usb: gadget: f_midi: free usb request when done

Hi Alan,

On Wed, Sep 23, 2015 at 3:30 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> On Wed, 23 Sep 2015, Felipe Tonello wrote:
>
>> Hi Peter,
>>
>> On Wed, Sep 23, 2015 at 4:10 AM, Peter Chen <peter.chen@...escale.com> wrote:
>> > On Tue, Sep 22, 2015 at 07:59:09PM +0100, Felipe F. Tonello wrote:
>> >> req->actual == req->length means that there is no data left to enqueue,
>> >> so free the request.
>> >>
>> >> Signed-off-by: Felipe F. Tonello <eu@...ipetonello.com>
>> >> ---
>> >>  drivers/usb/gadget/function/f_midi.c | 5 ++++-
>> >>  1 file changed, 4 insertions(+), 1 deletion(-)
>> >>
>> >> diff --git a/drivers/usb/gadget/function/f_midi.c b/drivers/usb/gadget/function/f_midi.c
>> >> index edb84ca..e92aff5 100644
>> >> --- a/drivers/usb/gadget/function/f_midi.c
>> >> +++ b/drivers/usb/gadget/function/f_midi.c
>> >> @@ -258,7 +258,10 @@ f_midi_complete(struct usb_ep *ep, struct usb_request *req)
>> >>               } else if (ep == midi->in_ep) {
>> >>                       /* Our transmit completed. See if there's more to go.
>> >>                        * f_midi_transmit eats req, don't queue it again. */
>> >> -                     f_midi_transmit(midi, req);
>> >> +                     if (req->actual < req->length)
>> >> +                             f_midi_transmit(midi, req);
>> >> +                     else
>> >> +                             free_ep_req(ep, req);
>> >>                       return;
>> >>               }
>> >
>> > It is incorrect, if no reqeust in queue, how device knows when
>> > the host sends data?
>>
>> This is the complete function of the IN endpoint.
>>
>> Actually I believe the proper patch is to enqueue this request again
>> if req->actual < req->length is true. Because the data is still there,
>> just not fully completed. Asking to transmit the request again will
>> cause to read new data from ALSA MIDI module, which it can possibly
>> steal data from a real ALSA request from f_midi_in_trigger. If that
>> doesn't happen (req->length == 0), the request will be freed anyway.
>>
>> Any thoughts?
>
> Please pardon me for jumping in in the middle of a conversation.  I
> know practically zero about f_midi.  But nevertheless...
>
> How can you ever have req->actual < req->length for a usb_request on an
> IN endpoint?  The only way that can happen is if some sort of error or
> exceptional event occurred, for example, if the transfer was cancelled
> before it could run to completion.  In such cases I doubt that you
> really want to retransmit the data.  Particularly since part of it
> probably was received by the host -- do you really want to send that
> part of the data a second time?

That is a fair point.

IMO we should always free the request upon a completion. Never
retransmit, since ALSA trigger will do that anyway.

Felipe
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