linux-kernel - Re: [PATCH] usb: gadget: f_midi: Fixed a bug when buflen was smaller than wMaxPacketSize

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Date:	Fri, 11 Mar 2016 18:07:10 -0500
From:	Michal Nazarewicz <mina86@...a86.com>
To:	"Felipe F. Tonello" <eu@...ipetonello.com>,
	linux-usb@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Felipe Balbi <balbi@...nel.org>,
	Robert Baldyga <r.baldyga@...sung.com>
Subject: Re: [PATCH] usb: gadget: f_midi: Fixed a bug when buflen was smaller than wMaxPacketSize

On Wed, Mar 09 2016, Felipe F. Tonello wrote:
> buflen by default (256) is smaller than wMaxPacketSize (512) in high-speed
> devices.
>
> That caused the OUT endpoint to freeze if the host send any data packet of
> length greater than 256 bytes.
>
> This is an example dump of what happended on that enpoint:
> HOST:   [DATA][Length=260][...]
> DEVICE: [NAK]
> HOST:   [PING]
> DEVICE: [NAK]
> HOST:   [PING]
> DEVICE: [NAK]
> ...
> HOST:   [PING]
> DEVICE: [NAK]
>
> This patch fixes this problem by setting the minimum usb_request's buffer size
> for the OUT endpoint as its wMaxPacketSize.
>
> Signed-off-by: Felipe F. Tonello <eu@...ipetonello.com>

Acked-by: Michal Nazarewicz <mina86@...a86.com>

But see comment below:

> ---
>  drivers/usb/gadget/function/f_midi.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/usb/gadget/function/f_midi.c b/drivers/usb/gadget/function/f_midi.c
> index 8475e3dc82d4..826ba641f29d 100644
> --- a/drivers/usb/gadget/function/f_midi.c
> +++ b/drivers/usb/gadget/function/f_midi.c
> @@ -366,7 +366,9 @@ static int f_midi_set_alt(struct usb_function *f, unsigned intf, unsigned alt)
>  	/* allocate a bunch of read buffers and queue them all at once. */
>  	for (i = 0; i < midi->qlen && err == 0; i++) {
>  		struct usb_request *req =
> -			midi_alloc_ep_req(midi->out_ep, midi->buflen);
> +			midi_alloc_ep_req(midi->out_ep,
> +				max_t(unsigned, midi->buflen,
> +					bulk_out_desc.wMaxPacketSize));

Or, just:

+			midi_alloc_ep_req(midi->out_ep,
+					  bulk_out_desc.wMaxPacketSize);

Packet cannot be greater than wMaxPacketSize so there is no need to
allocate more (if buflen > wMaxPacketSize).

>  		if (req == NULL)
>  			return -ENOMEM;
>  

I’m also wondering whether it would be beneficial to get rid of buflen
all together and use wMaxPacketSie for in endpoints as well?  Is that
feasible?

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ
«If at first you don’t succeed, give up skydiving»