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

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Date:	Tue, 19 Jul 2016 11:30:27 +0300
From:	Felipe Balbi <felipe.balbi@...ux.intel.com>
To:	Bin Gao <bin.gao@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	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


Hi,

Bin Gao <bin.gao@...ux.intel.com> writes:
> 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.

what Greg is saying is that you should register yourself to the PD stack
with something that passes along a pointer to the actual device.

> 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.

This is the wrong analogy.

-- 
balbi

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