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From:   "Yoshida, Shigeru" <Shigeru.Yoshida@windriver.com>
To:     "stern@rowland.harvard.edu" <stern@rowland.harvard.edu>
CC:     "Bai, Haiqing" <Haiqing.Bai@windriver.com>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] ohci-hcd: Fix race condition caused by
 ohci_urb_enqueue() and io_watchdog_func()
Thread-Topic: [PATCH] ohci-hcd: Fix race condition caused by
 ohci_urb_enqueue() and io_watchdog_func()
Thread-Index: AQHTmncRqxjtZV1slkO7r5QpQNTFa6OOqw2AgADm5YCAAKNFgIAAq14A
Date:   Fri, 2 Feb 2018 01:46:01 +0000
Message-ID: <20180202.104654.1854598770724026730.shigeru.yoshida@windriver.com>
References: <20180201.144911.2171407625301691891.shigeru.yoshida@windriver.com>
        <Pine.LNX.4.44L0.1802011029160.1677-100000@iolanthe.rowland.org>
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Hi Alan,

On Thu, 1 Feb 2018 10:33:33 -0500, Alan Stern wrote:
>> I think we must choose an invalid frame number for the special
>> sentinel value, but I'm not sure which value is adequate for it.
>> Is 0xffffff00 an invalid frame number, otherwise how about simply
>> -1(0xffffffff)?
> 
> Well, the frame_no register is 32 bits wide, but only the 16 low-order 
> bits are meaningful.  ohci_frame_no() strips off the high-order 16 
> bits, so any value with one of those bits set would be acceptable.  
> (Besides, valid frame numbers only go up to 2047.)
> 
> I chose 0xffffff00 because PCI reads from a non-working device 
> generally get a value with all the bits set.  But since the upper 16 
> bits are masked away anyhow, it doesn't matter.  -1u would be fine.

Thanks, I choose 0xffffff00 as you suggested, and will prepare v2 patch.

Thanks,
Shigeru