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

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Date:   Tue, 30 May 2017 15:46:25 +0200
From:   Vlastimil Babka <vbabka.lkml@...il.com>
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>
Subject: Re: [PATCH v8 2/5] usb: early: add driver for xhci debug capability

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.

Thanks,
Vlastimil