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

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Date:	Wed, 30 Mar 2016 13:51:17 +0300
From:	Felipe Balbi <balbi@...nel.org>
To:	Michal Nazarewicz <mina86@...a86.com>,
	"Felipe F. Tonello" <eu@...ipetonello.com>,
	linux-usb@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	Robert Baldyga <r.baldyga@...sung.com>
Subject: Re: [PATCH] usb: gadget: f_midi: Fixed a bug when buflen was smaller than wMaxPacketSize

Michal Nazarewicz <mina86@...a86.com> writes:
> [ text/plain ]
> 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).

a USB packet, right. that's correct. But a struct usb_request can point
to whatever size buffer it wants and UDC is required to split that into
wMaxPacketSize transfers.

-- 
balbi

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