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

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Date:	Fri, 15 Jul 2016 13:38:12 +0300
From:	Felipe Balbi <felipe.balbi@...ux.intel.com>
To:	Bin Gao <bin.gao@...ux.intel.com>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:	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

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.

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

-- 
balbi

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