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

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Date:   Thu, 1 Jun 2017 10:15:24 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Lu Baolu <baolu.lu@...ux.intel.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>,
        Michal Kubecek <mkubecek@...e.cz>
Subject: Re: [PATCH v8 2/5] usb: early: add driver for xhci debug capability

On 06/01/2017 05:37 AM, Lu Baolu wrote:
> 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:
>>>> 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.

Thanks. I didn't make it clear that the trace_printk() warning is there
even if the code using it doesn't actually execute (i.e. I didn't
specify any early_printk bootparam). There are some roastedy tricks to
detect the potential users, so that the buffers can be allocated in
advance to allow the first trace_printk() from any context, I guess.

I'm not sure if there's a way to change it so that your driver reports
the trace_printk usage only in response to the bootparam (which could
also be a safe point to allocate ftrace buffers?).

Vlastimil