RE: [PATCH v7 01/10] usb: gadget: udc: Add timer support for usb requests

From: Felipe Balbi <balbi@kernel.org>
To: Anurag Kumar Vulisha <anuragku@xilinx.com>,
	Alan Stern <stern@rowland.harvard.edu>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Shuah Khan <shuah@kernel.org>, Johan Hovold <johan@kernel.org>,
	Jaejoong Kim <climbbb.kim@gmail.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Roger Quadros <rogerq@ti.com>,
	Manu Gautam <mgautam@codeaurora.org>,
	"martin.petersen\@oracle.com" <martin.petersen@oracle.com>,
	Bart Van Assche <bvanassche@acm.org>,
	Mike Christie <mchristi@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Colin Ian King <colin.king@canonical.com>,
	"linux-usb\@vger.kernel.org" <linux-usb@vger.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"v.anuragkumar\@gmail.com" <v.anuragkumar@gmail.com>,
	Thinh Nguyen <thinhn@synopsys.com>,
	Tejas Joglekar <tejas.joglekar@synopsys.com>,
	Ajay Yugalkishore Pandey <APANDEY@xilinx.com>
Subject: RE: [PATCH v7 01/10] usb: gadget: udc: Add timer support for usb requests
Date: Fri, 07 Dec 2018 08:05:39 +0200	[thread overview]
Message-ID: <877eglx45o.fsf@linux.intel.com> (raw)
In-Reply-To: <BL0PR02MB563386B88BDC306AF44EE736A7A80@BL0PR02MB5633.namprd02.prod.outlook.com>

hi,

Anurag Kumar Vulisha <anuragku@xilinx.com> writes:
>>Does the data book suggest a value for the timeout?
>>
>
> No, the databook doesn't mention about the timeout value
>
>>> >At this point, it seems that the generic approach will be messier than having every
>>> >controller driver implement its own fix.  At least, that's how it appears to me.

Why, if the UDC implementation will, anyway, be a timer?

>>(Especially if dwc3 is the only driver affected.)
>
> As discussed above, the issue may happen with other gadgets too. As I got divide opinions
> on this implementation and both the implementations looks fine to me, I am little confused
> on which should be implemented.
>
> @Felipe: Do you agree with Alan's implementation? Please let us know your suggestion
> on this.

I still think a generic timer is a better solution since it has other uses.

>>> >Ideally it would not be necessary to rely on a timeout at all.
>>> >
>>> >Also, maintainers dislike module parameters.  It would be better not to add one.
>>>
>>> Okay. I would be happy if any alternative for this issue is present but unfortunately
>>> I am not able to figure out any alternative other than timers. If not
>>module_params()
>>> we can add an configfs entry in stream gadget to update the timeout. Please
>>provide
>>> your opinion on this approach.
>>
>>Since the purpose of the timeout is to detect a deadlock caused by a
>>hardware bug, I suggest a fixed and relatively short timeout value such
>>as one second.  Cancelling and requeuing a few requests at 1-second
>>intervals shouldn't add very much overhead.

I wouldn't call this a HW bug though. This is just how the UDC
behaves. There are N streams and host can move data in any stream at any
time. This means that host & gadget _can_ disagree on what stream to
start next.

One way to avoid this would be to never pre-start any streams and always
rely on XferNotReady, but that would mean greatly reduced throughput for
streams.

-- 
balbi