linux-kernel - RE: [PATCH v1 2/2] usb:cdns3 Add Cadence USB3 DRD Driver

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Date:   Thu, 13 Dec 2018 01:21:29 +0000
From:   Peter Chen <peter.chen@....com>
To:     Bin Liu <b-liu@...com>, Peter Chen <hzpeterchen@...il.com>
CC:     "pawell@...ence.com" <pawell@...ence.com>,
        "balbi@...nel.org" <balbi@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "rogerq@...com" <rogerq@...com>,
        lkml <linux-kernel@...r.kernel.org>,
        "adouglas@...ence.com" <adouglas@...ence.com>,
        "jbergsagel@...com" <jbergsagel@...com>,
        "nsekhar@...com" <nsekhar@...com>, "nm@...com" <nm@...com>,
        "sureshp@...ence.com" <sureshp@...ence.com>,
        "pjez@...ence.com" <pjez@...ence.com>,
        "kurahul@...ence.com" <kurahul@...ence.com>
Subject: RE: [PATCH v1 2/2] usb:cdns3 Add Cadence USB3 DRD Driver

 
> >
> > Felipe, I use Dynamic Debug for debugging, and show debug messages
> > with "dmesg" after testing/debugging. I see dwc3 using trace,  any
> > benefits for switching to trace?
> 
> The benefits I see are
> 

Thanks, bin.

> - *by default*, the debug log doesn't have to go through uart console,
>   which is slow.
> 
>   I typically 'cat trace_pipe' from a telnet which is much faster then
>   uart. But Dynamic Debug log by default got printed on uart, I have to
>   set printk level to not print them, which is an extra step.
> 

It depends on the rootfs's setting. By default, the debug message level should be off (
console level = 7)

> - tracepoint uses one place to decode the message vs DD has to repeat
>   the similar print statement in the code; and tracepoint decodes it
>   offline which reduce the kernel runtime overhead too.
> 

offline? You mean when you run "cat trace_pipe"?

> - with tracepoint, it is easier to turn on debug for a specific group of
>   messages.
> 

The DD can, but we don't do it often.

https://www.kernel.org/doc/html/latest/admin-guide/dynamic-debug-howto.html?highlight=dynamic%20debug

> - you can adjust the ftrace buffer size at runtime. can we do that for
>   printk? I don't remember.

It may can't. The log buf can only be changed by bootargs: log_buf_len

Peter