linux-kernel - Re: [PATCH v3 1/4] usb: gadget: f_midi: free usb request when done

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Message-ID: <56178799.9000409@ladisch.de>
Date:	Fri, 9 Oct 2015 11:23:37 +0200
From:	Clemens Ladisch <clemens@...isch.de>
To:	Felipe Tonello <eu@...ipetonello.com>,
	USB list <linux-usb@...r.kernel.org>
Cc:	Kernel development list <linux-kernel@...r.kernel.org>,
	Peter Chen <Peter.Chen@...escale.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Felipe Balbi <balbi@...com>,
	Andrzej Pietrasiewicz <andrzej.p@...sung.com>
Subject: Re: [PATCH v3 1/4] usb: gadget: f_midi: free usb request when done

Felipe Tonello wrote:
> req->actual == req->length means that there is no data left to enqueue,

This condition is not checked in the patch.

> so free the request.
>
> Signed-off-by: Felipe F. Tonello <eu@...ipetonello.com>
> ---
>  drivers/usb/gadget/function/f_midi.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> diff --git a/drivers/usb/gadget/function/f_midi.c b/drivers/usb/gadget/function/f_midi.c
> index edb84ca..93212ca 100644
> --- a/drivers/usb/gadget/function/f_midi.c
> +++ b/drivers/usb/gadget/function/f_midi.c
> @@ -256,9 +256,8 @@ f_midi_complete(struct usb_ep *ep, struct usb_request *req)
>  			/* We received stuff. req is queued again, below */
>  			f_midi_handle_out_data(ep, 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);
> +			/* Our transmit completed. Don't queue it again. */
> +			free_ep_req(ep, req);
>  			return;
>  		}
>  		break;

The ALSA framework will give you a notification _once_.  If the
resulting data is larger than midi->buflen, then you have to continue
sending packets.  This is exactly what the call to f_midi_transmit()
does.

(To decrease latency, it might be a good idea to queue multiple requests,
but you wouldn't want to continue piling up requests if the host isn't
listening.  sound/usb/midi.c just allocates a fixed number of requests,
and always reuses them.)


Regards,
Clemens
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