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 S1754711AbcKJBNV (ORCPT <rfc822;w@1wt.eu>);
        Wed, 9 Nov 2016 20:13:21 -0500
Received: from mga05.intel.com ([192.55.52.43]:33026 "EHLO mga05.intel.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1752580AbcKJBNU (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 9 Nov 2016 20:13:20 -0500
X-ExtLoop1: 1
X-IronPort-AV: E=Sophos;i="5.31,616,1473145200"; 
   d="scan'208";a="1083123152"
Subject: Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability
To: Thomas Gleixner <tglx@linutronix.de>
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.2.20.1611091015170.3501@nanos>
Cc: 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>,
        Peter Zijlstra <peterz@infradead.org>
From: Lu Baolu <baolu.lu@linux.intel.com>
Message-ID: <5823C9AC.7050209@linux.intel.com>
Date: Thu, 10 Nov 2016 09:13:16 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.5.1
MIME-Version: 1.0
In-Reply-To: <alpine.DEB.2.20.1611091015170.3501@nanos>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Hi,

On 11/09/2016 05:23 PM, Thomas Gleixner wrote:
> On Tue, 1 Nov 2016, Lu Baolu wrote:
>> +static int __init xdbc_init(void)
>> +{
> ...
>> +	base = ioremap_nocache(xdbc.xhci_start, xdbc.xhci_length);
>> +	if (!base) {
>> +		xdbc_trace("failed to remap the io address\n");
>> +		ret = -ENOMEM;
>> +		goto free_and_quit;
>> +	}
>> +
>> +	early_iounmap(xdbc.xhci_base, xdbc.xhci_length);
>> +	xdbc_trace("early mapped IO address released\n");
>> +
>> +	xdbc.xhci_base = base;
>> +	offset = xhci_find_next_ext_cap(xdbc.xhci_base, 0, XHCI_EXT_CAPS_DEBUG);
>> +	xdbc.xdbc_reg = (struct xdbc_regs __iomem *)(xdbc.xhci_base + offset);
> This is broken. What prevents that 
>
>      - a printk is in progress on another cpu?
>
>      - a printk happens between the unmap and storing the new base ?
>
> Nothing AFAICT. So this needs to be done in a safe way. And just making it
>
> 	oldbase = xdbc.xhci_base;
> 	base = ioremap(....);
> 	xdbc.xhci_base = base;
> 	early_iounmap(oldbase);
>
> does not work either because the compiler can rightfully cache
> xdbc.xhci_base in the write related functions. The same issue with
> xdbc.xdbc_reg.

If there isn't a good solution, I will remove the ioremap code and let
it use the early mapped one.

Best regards,
Lu Baolu