linux-kernel - Re: [PATCH 1/2] usb: typec: Add USB Power Delivery sink port support

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Date:	Fri, 15 Jul 2016 20:11:41 +0900
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Felipe Balbi <felipe.balbi@...ux.intel.com>
Cc:	Bin Gao <bin.gao@...ux.intel.com>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	Bin Gao <bin.gao@...el.com>,
	Chandra Sekhar Anagani <chandra.sekhar.anagani@...el.com>
Subject: Re: [PATCH 1/2] usb: typec: Add USB Power Delivery sink port support

On Fri, Jul 15, 2016 at 01:38:12PM +0300, Felipe Balbi wrote:
> 
> Hi,
> 
> Bin Gao <bin.gao@...ux.intel.com> writes:
> > +static void print_message(int port, bool is_cmsg, u8 msg, bool recv)
> > +{
> > +	pr_info("sink port %d: %s message %s %s\n", port,
> > +				is_cmsg ? "Control" : "Data",
> > +				msg_to_string(is_cmsg, msg),
> > +		 recv ? "received" : "sent(wait GOODCRC)");
> > +}
> 
> this is problematic. By default, we're all using 115200 8N1 baud
> rate. This message alone prints anywhere from 50 to 100 characters (I
> didn't really count properly, these are rough numbers), and that takes:
> 
> n50chars_time = 50 / (115200 / 10) = 4.3ms
> n100chars_time = 100 / (115200 / 10) = 8.6ms
> 
> Considering you have 30ms to reply with Power Request after GoodCRC, and
> considering you're printing several of these messages, they become
> really expensive and eat up valuable time from tSenderReply.

printk() should be async, so it shouldn't be that big of a deal.

What is wrong is that this isn't using dev_info().

> This should really be a pr_debug() or, better yet, a tracepoint.

Yes, that would be best (dev_dbg() or a tracepoint.)

thanks,

greg k-h