linux-kernel - Re: usb: gadget: f_fs: O_NONBLOCK waits MAX_SCHEDULE

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Date:	Fri, 22 May 2015 12:12:41 +0100
From:	"Baxter, Jim" <jim_baxter@...tor.com>
To:	Peter Chen <peter.chen@...escale.com>
CC:	Robert Baldyga <r.baldyga@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"Zapolskiy, Vladimir" <Vladimir_Zapolskiy@...tor.com>,
	"balbi@...com" <balbi@...com>
Subject: Re: usb: gadget: f_fs: O_NONBLOCK waits MAX_SCHEDULE_TIMEOUT


> On Wed, Apr 01, 2015 at 06:29:05PM +0100, Baxter, Jim wrote:
>>>
>>> FunctionFS is very specific, because read/write operations are directly
>>> translated into USB requests, which are asynchronous, so you cannot use
>>> O_NONBLOCK.
>>>
>>> If you need non-blocking API you can use Asynchronous I/O (AIO). You can
>>> find some examples in kernel sources (tools/usb/ffs-aio-example/).
>>>
>>> Br,
>>> Robert Baldyga
>>>
>>
>> Thank you, that sounds like the best approach.
>> In this case I think perhaps the long wait without any data is an
>> problem with the imx6 Chipidea USB controller.
> 
> What's the possible problem?

Sorry for the delay in replying, I have been getting some more details
with a USB Analyser.

The scenario is that the NCM  device is enumerating so we see the
messages to:

SetAddress (1)
GetDescriptor (Device)
GetDescriptor (StringN)
GetDescriptor (Configuration)
SetConfiguration (1)
GetDescriptor (String iInterface)
GetDescriptor (String iInterface)

At this point the NCM host sends Writes to the F_FS EP0 but for some
reason the host device does not respond and only issues SOF packets for
hours. This happens occasionally and is fixed by turning the device off
and on again.


Unless I am mistaken from a NCM gadget point of view the attached device
is working correctly and there is no way to know it has failed, is that
correct?

> 
>>
>> I guess it should suspend and drop the connections if there is no
>> traffic for more than 10ms?
>>
> 
> If the Device side NAK host's IN/OUT token continually, the pipe will 
> not be stopped, the host will send token continually until the application
> cancel this request.
> 
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