linux-kernel - Re: [PATCH v5 1/4] usb: dbc: early driver for xhci debug capability

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Message-ID: <589C1C35.2070604@linux.intel.com>
Date:   Thu, 9 Feb 2017 15:37:25 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, tglx@...utronix.de,
        linux-usb@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH v5 1/4] usb: dbc: early driver for xhci debug capability

Hi Ingo,

On 01/26/2017 03:22 PM, Ingo Molnar wrote:
> * Lu Baolu <baolu.lu@...ux.intel.com> wrote:
>
>> Hi,
>>
>> On 01/25/2017 10:38 PM, Peter Zijlstra wrote:
>>> On Wed, Jan 25, 2017 at 08:27:38PM +0800, Lu Baolu wrote:
>>>> In my driver, udelay() is mostly used to handle time out.
>>>>
>>>> Xdbc hides most USB things in its firmware. Early printk driver only needs
>>>> to setup the registers/data structures and wait until link ready or time out.
>>>> Without udelay(), I have no means to convert the polling times into waiting
>>>> time.
>>> What is timeout and why?
>> Put it in simple:
>>
>> The driver sets the RUN bit in control register and polls READY
>> bit in status register for the successful USB device enumeration.
>> As the USB device enumeration might fail and the READY bit will
>> never be set, the driver must have a timeout logic to avoid
>> endless loop.
> Is there any error status available in the host registers anywhere that tells us 
> that enumeration did not succeed?

No, there isn't. The xhci spec requires software to impose a timeout.

Page 425, xhci specification:

"
Software shall impose a timeout between the detection
of the Debug Host connection and the DbC Run transition
to ‘1’.
"

Best regards,
Lu Baolu