linux-kernel - Re: [PATCH] usb: dwc3: debugfs: add snapshot to dump requests trbs events

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Message-ID: <20140428235957.GD18010@saruman.home>
Date:	Mon, 28 Apr 2014 18:59:57 -0500
From:	Felipe Balbi <balbi@...com>
To:	Paul Zimmerman <Paul.Zimmerman@...opsys.com>
CC:	"balbi@...com" <balbi@...com>,
	Zhuang Jin Can <jin.can.zhuang@...el.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: dwc3: debugfs: add snapshot to dump requests trbs
 events

On Mon, Apr 28, 2014 at 07:58:43PM +0000, Paul Zimmerman wrote:
> > From: linux-usb-owner@...r.kernel.org [mailto:linux-usb-owner@...r.kernel.org] On Behalf Of Felipe Balbi
> > Sent: Monday, April 28, 2014 8:56 AM
> > 
> > On Mon, Apr 28, 2014 at 04:49:23PM -0400, Zhuang Jin Can wrote:
> > > Adds a debugfs file "snapshot" to dump dwc3 requests, trbs and events.
> 
> < snip > 
> 
> > > +	for (i = 0; i < evt->length; i += 4) {
> > > +		event.raw = *(u32 *) (evt->buf + i);
> > > +		if (i == evt->lpos) {
> > > +			seq_puts(s, "lpos-------|\n");
> > > +			seq_puts(s, "           \\\n");
> > > +		}
> > > +		seq_printf(s, "event[%d]: %08x ", i, event.raw);
> > > +
> > > +		if (event.type.is_devspec)
> > > +			dwc3_dump_dev_event(s, &event.devt);
> > > +		else
> > > +			dwc3_dump_ep_event(s, &event.depevt);
> > > +		seq_puts(s, "\n");
> > > +	}
> > 
> > how well have you tested this ? I'm not entirely sure you should be
> > reading the event buffer at any time you want, though I might be wrong.
> 
> Just FYI, it is fine to read from the event buffer memory at any time.
> But of course, if you happen to read from a location at the same time
> the core is writing to it, you may read either the old value or the new,
> depending on the timing. So the results might not be 100% reliable.

Thanks, based on this I'll stick to my
"use-a-ring-buffer-to-store-last-5-events" suggestion :-)

-- 
balbi

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