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

From: Lu Baolu <baolu.lu@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Mathias Nyman <mathias.nyman@linux.intel.com>,
	Ingo Molnar <mingo@redhat.com>,
	tglx@linutronix.de, linux-usb@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org, Jiri Slaby <jslaby@suse.cz>
Subject: Re: [PATCH v5 1/4] usb: dbc: early driver for xhci debug capability
Date: Wed, 25 Jan 2017 23:51:34 +0800	[thread overview]
Message-ID: <5888C986.4020809@linux.intel.com> (raw)
In-Reply-To: <20170125143829.GS6515@twins.programming.kicks-ass.net>

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.

More details:

The operational model is that driver sets up all necessary registers
and data structures, and then starts the debug engine by setting
the RUN/STOP bit in the control register.

The debug engine then brings up itself as a ready-for-enumeration
USB device. The USB link between host and device starts link training
and then host will detect the connected device. The hub driver in
host will then starts the USB device enumeration processes (as defined
in USB spec). If everything goes smoothly, the device gets enumerated
and host can talk with the debug device.

After that, xdbc firmware will set the READY bit in status register. And
the driver can go ahead with data transfer over USB.

>  If there is an error other than !ready, I would
> expect the hardware to inform you of this through another status bit,
> no?

Yeah, this might be another choice of hardware design. But it's not a
topic for this driver.

>
> So why can't you poll indefinitely for either ready or error?
>
>

Even if the hardware has both ready and error status bits, it's still
nice to have a time out watch dog. Buggy hardware or firmware
might not set any of these bits. Polling indefinitely might result in
a endless loop.

Best regards,
Lu Baolu