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

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Message-ID: <20170126072219.GB3399@gmail.com>
Date:   Thu, 26 Jan 2017 08:22:19 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
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


* 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?

Thanks,

	Ingo