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

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Message-ID: <20160719053038.GA90550@worksta>
Date:	Mon, 18 Jul 2016 22:30:38 -0700
From:	Bin Gao <bin.gao@...ux.intel.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Felipe Balbi <felipe.balbi@...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 Sat, Jul 16, 2016 at 08:49:53AM +0900, Greg Kroah-Hartman wrote:
> On Fri, Jul 15, 2016 at 03:41:10PM -0700, Bin Gao wrote:
> > On Fri, Jul 15, 2016 at 02:21:48PM +0300, Felipe Balbi wrote:
> > > Greg Kroah-Hartman <gregkh@...uxfoundation.org> writes:
> > > > 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.
> > > 
> > > I can actually see this causing problems ;-) With this pr_info(),
> > > sometimes tSenderReply times out and Source gives a HardReset. Without
> > > pr_info(), type-c analyzer tells me we reply in less than 1ms.
> > > 
> > > > What is wrong is that this isn't using dev_info().
> > > 
> > > right, that too.
> > > 
> > > -- 
> > > balbi
> > 
> > When we don't have a struct device pointer for this driver,
> 
> Then you should fix that, as this is a driver for hardware :)
This is actualy a software stack to implement the USB PD spec.
Only the USB Type-C phy driver has a device pointer.
The PD stack vs. USB Type-C phy driver is similar to TCP/IP stack
vs. ethernet driver in the kernel. We don't have a device pointer
for TCP/IP stack code either.

Thanks,
Bin

> 
> thanks,
> 
> greg k-h