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

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Message-ID: <592F8BF3.4070004@linux.intel.com>
Date:   Thu, 1 Jun 2017 11:37:23 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        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>,
        Michal Kubecek <mkubecek@...e.cz>
Subject: Re: [PATCH v8 2/5] usb: early: add driver for xhci debug capability

Hi,

On 05/31/2017 05:38 PM, Vlastimil Babka wrote:
> On 05/31/2017 05:27 AM, Lu Baolu wrote:
>> 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.
> Hmm, but it seems we generally enable all these early printk
> features/drivers in our distro kernels. They are not active without a
> early_printk=X bootparam anyway, right? It's much more convenient to
> e.g. just tell customer to change a param when debugging something than
> to install a debug kernel. So I wouldn't agree that only a DEBUG kernel
> should have this compiled in.

Okay, I will try to find a fix.

Best regards,
Lu Baolu

>
> Thanks,
> Vlastimil
>
>> Best regards,
>> Lu Baolu
>>
>>> Thanks,
>>> Vlastimil
>>>
> --
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