linux-kernel - Re: [PATCH v8 2/5] usb: early: add driver for xhci debug capability

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Date:   Wed, 31 May 2017 11:27:19 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Vlastimil Babka <vbabka.lkml@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Mathias Nyman <mathias.nyman@...ux.intel.com>, tglx@...utronix.de,
        peterz@...radead.org, linux-usb@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v8 2/5] usb: early: add driver for xhci debug capability

Hi,

On 05/30/2017 09:46 PM, Vlastimil Babka wrote:
> On 03/21/2017 09:01 AM, Lu Baolu wrote:
>> XHCI debug capability (DbC) is an optional but standalone
>> functionality provided by an xHCI host controller. Software
>> learns this capability by walking through the extended
>> capability list of the host. XHCI specification describes
>> DbC in the section 7.6.
>>
>> This patch introduces the code to probe and initialize the
>> debug capability hardware during early boot. With hardware
>> initialized, the debug target (system on which this code is
>> running) will present a debug device through the debug port
>> (normally the first USB3 port). The debug device is fully
>> compliant with the USB framework and provides the equivalent
>> of a very high performance (USB3) full-duplex serial link
>> between the debug host and target. The DbC functionality is
>> independent of the xHCI host. There isn't any precondition
>> from the xHCI host side for the DbC to work.
>>
>> One use for this feature is kernel debugging, for example
>> when your machine crashes very early before the regular
>> console code is initialized. Other uses include simpler,
>> lockless logging instead of a full-blown printk console
>> driver and klogd.
>>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Mathias Nyman <mathias.nyman@...ux.intel.com>
>> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ...
>
>> +
>> +#define XDBC_TRACE
>> +#ifdef XDBC_TRACE
>> +#define	xdbc_trace	trace_printk
> Did you forget to remove the #define XDBC_TRACE?
>
> Enabling this driver brings the "trace_printk() being used. Allocating
> extra memory. This means that this is a DEBUG kernel and it is unsafe
> for production use." message in 4.12-rcX dmesg.

This feature is only for a DEBUG kernel, should not be enabled for
any production kernel. This was the reason I enabled trace_printk()
by default.

Best regards,
Lu Baolu

> Thanks,
> Vlastimil
>