From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1755524AbcKJLoq (ORCPT <rfc822;w@1wt.eu>);
        Thu, 10 Nov 2016 06:44:46 -0500
Received: from merlin.infradead.org ([205.233.59.134]:53250 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1755362AbcKJLoo (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 10 Nov 2016 06:44:44 -0500
Date: Thu, 10 Nov 2016 12:44:45 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Lu Baolu <baolu.lu@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mathias Nyman <mathias.nyman@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, linux-usb@vger.kernel.org,
        x86@kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability
Message-ID: <20161110114445.GW3142@twins.programming.kicks-ass.net>
References: <1477976354-13291-1-git-send-email-baolu.lu@linux.intel.com>
 <1477976354-13291-2-git-send-email-baolu.lu@linux.intel.com>
 <alpine.DEB.2.20.1611091030171.3501@nanos>
 <5823CB63.7090603@linux.intel.com>
 <alpine.DEB.2.20.1611100946190.3501@nanos>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <alpine.DEB.2.20.1611100946190.3501@nanos>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Nov 10, 2016 at 09:56:41AM +0100, Thomas Gleixner wrote:
> On Thu, 10 Nov 2016, Lu Baolu wrote:

> > This seems to be a common issue for all early printk drivers.
> 
> No. The other early printk drivers like serial do not have that problem as
> they simply do:
> 
>    while (*buf) {
>       while (inb(UART) & TX_BUSY)
>    	 cpu_relax();
>       outb(*buf++, UART);
>    }

Right, which is why actual UARTs rule. If only laptops still had pinouts
for them life would be sooooo much better.

Ideally the USB debug port would be a virtual UART and its interface as
simple and robust.

> The wait for the UART to become ready is independent of the context as it
> solely depends on the hardware.
> 
> As a result you can see the output from irq/nmi intermingled with the one
> from thread context, but that's the only problem they have.
> 
> The only thing you can do to make this work is to prevent printing in NMI
> context:
> 
> write()
> {
> 	if (in_nmi())
> 		return;
> 	
> 	raw_spinlock_irqsave(&lock, flags);
> 	....
> 
> That fully serializes the writes and just ignores NMI context printks. Not
> optimal, but I fear that's all you can do.

I would also suggest telling the hardware people they have designed
something near the brink of useless. If you cannot do random exception
context debugging (#DB, #NMI, #MCE etc..) then there's a whole host of
problems that simply cannot be debugged.

Also note that kdb runs from NMI context, so you'll not be able to
support that either.