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Date: Sun, 22 Jan 2017 10:04:23 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mathias Nyman <mathias.nyman@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, tglx@linutronix.de,
        peterz@infradead.org, linux-usb@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 1/4] usb: dbc: early driver for xhci debug capability
Message-ID: <20170122090423.GA15061@gmail.com>
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* Lu Baolu <baolu.lu@linux.intel.com> wrote:

> >
> >> +static void xdbc_runtime_delay(unsigned long count)
> >> +{
> >> +	udelay(count);
> >> +}
> >> +static void (*xdbc_delay)(unsigned long) = xdbc_early_delay;
> > Is this udelay() complication really necessary? udelay() should work fine even in 
> > early code. It might not be precisely calibrated, but should be good enough.
> 
> I tried udelay() in the early code. It's not precise enough for the
> hardware handshaking.

Possibly because on x86 early udelay() did not work at all - i.e. there's no delay 
whatsoever.

Could you try it on top of this commit in tip:timers/core:

  4c45c5167c95 x86/timer: Make delay() work during early bootup

?

Thanks,

	Ingo