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

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Message-ID: <Pine.LNX.4.44L0.1812030945440.21114-100000@netrider.rowland.org>
Date:   Mon, 3 Dec 2018 09:51:01 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Anurag Kumar Vulisha <anuragku@...inx.com>
cc:     Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shuah Khan <shuah@...nel.org>, Johan Hovold <johan@...nel.org>,
        Jaejoong Kim <climbbb.kim@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Roger Quadros <rogerq@...com>,
        Manu Gautam <mgautam@...eaurora.org>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        Bart Van Assche <bvanassche@....org>,
        Mike Christie <mchristi@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Colin Ian King <colin.king@...onical.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "v.anuragkumar@...il.com" <v.anuragkumar@...il.com>,
        Thinh Nguyen <thinhn@...opsys.com>,
        Tejas Joglekar <tejas.joglekar@...opsys.com>,
        Ajay Yugalkishore Pandey <APANDEY@...inx.com>
Subject: RE: [PATCH v7 01/10] usb: gadget: udc: Add timer support for usb
 requests

On Mon, 3 Dec 2018, Anurag Kumar Vulisha wrote:

> >First of all, if some sort of deadlock causes a transfer to fail to
> >complete, the host is expected to cancel and restart it.  Not the
> >gadget.
> >
> 
> Thanks for spending your time in reviewing this patch.  The deadlock
> is  a very rare case scenario and is happening because both the gadget
> controller & host controllers get out of sync and are stuck waiting for the
> relevant event. For example this issue is observed in stream protocol where
> the gadget controller is waiting on Host controller to issue PRIME transaction
> and  Host controller is waiting on gadget to issue ERDY transaction. Since
> the stream protocol is gadget driven, the host may not proceed further until it
> receives a valid Start Stream (ERDY) transaction from gadget.

That's not entirely true.  Can't the host cancel the transfer and then 
restart it?

> Since the gadget
> controller driver is aware that the controller is stuck , makes it responsible
> to recover the controller from hang condition by restarting the transfer (which
> triggers the controller FSM to issue ERDY to host).

Isn't there a cleaner way to recover than by cancelling the request and 
resubmitting it?

> >Second, if a request timer expires and the request is cancelled, the
> >gadget driver's completion handler will be called.  This is not what
> >you want if the UDC core is going to resubmit the request
> >automatically.
> >
> >Third, if a request timer expires and the timer handler calls
> >usb_ep_dequeue() followed immediately by usb_ep_queue_timeout(), the
> >resubmit will probably fail because the dequeue won't have completed
> >yet.
> >
> >Fourth, the patch contains a race between the timer expiring and the
> >request completing.
> 
> Thanks for correcting, I agree with you on all the above 3 cases that the
> resubmission of the request should only be done from the class driver and 
> the udc core should simply dequeue the request on timeout. I am not sure
> why I haven't seen any issue while testing on this patch series. I will modify
> the code to handle the resubmitting of requests properly.

How can the gadget driver know what timeout to use?  The host is
allowed to be as slow as it wants; the gadget driver doesn't have any
way to tell when the host wants to start the transfer.

Alan Stern