Message-ID: <2736758.WIzxm4p4Im@thinkpad>
Date:	Fri, 12 Jul 2013 12:34:25 +0200
From:	Tomasz Figa <tomasz.figa@...il.com>
To:	Jingoo Han <jg1.han@...sung.com>
Cc:	'Dongjin Kim' <tobetter@...il.com>,
	'Julius Werner' <jwerner@...omium.org>,
	'LKML' <linux-kernel@...r.kernel.org>,
	'Felipe Balbi' <balbi@...com>, linux-usb@...r.kernel.org,
	devicetree-discuss@...ts.ozlabs.org,
	linux-samsung-soc@...r.kernel.org,
	'Vivek Gautam' <gautam.vivek@...sung.com>,
	'Praveen Paneri' <p.paneri@...sung.com>,
	'Kukjin Kim' <kgene.kim@...sung.com>,
	'Tushar Behera' <tushar.behera@...aro.org>,
	'Doug Anderson' <dianders@...omium.org>,
	'Olof Johansson' <olofj@...omium.org>,
	'Vincent Palatin' <vpalatin@...omium.org>,
	'Thomas Abraham' <thomas.abraham@...aro.org>,
	'Fabio Estevam' <festevam@...il.com>,
	'Philipp Zabel' <p.zabel@...gutronix.de>,
	Yulgon Kim <yulgon.kim@...sung.com>, s.nawrocki@...sung.com,
	grant.likely@...aro.org, arnd@...db.de
Subject: Re: [PATCH] usb: phy: samsung-usb2: Toggle HSIC GPIO from device tree

Hi,

On Friday 12 of July 2013 09:48:54 Jingoo Han wrote:
> On Friday, July 12, 2013 6:46 AM, Julius Werner wrote:
> > Hi Jingoo,
> > 
> > Yeah, I followed that discussion back then, but it seems to have
> > stalled a little (at least the HSIC patches haven't been picked up in
> > any kernel.org repo yet to my knowledge).
> > 
> > The problem is that I think these approaches cannot work reliably. I
> > agree that it would be nice to control the HSIC device from its own
> > driver, and have spent quite some time playing around with the
> > usb/misc/usb3503.c driver to try to make this work... but there's a
> > timing dependency here that you just can't model correctly with
> > independent drivers.
> > 
> > If the HSIC device is already active during boot (e.g. because it was
> > used by firmware), there's always a chance that the USB stack will
> > come up before the driver that resets it does. The device will be
> > enumerated as normal, and when the other driver later pulls the reset
> > signal the USB stack will not notice because there is no real
> > disconnect detection on HSIC. Only when you eventually try to send
> > another transfer to the device will you start to get timeouts, and the
> > newly reset device will not be able to reenumerate because the host
> > never asks it to.
> > 
> > I really don't see how you could solve this without putting some kind
> > of synchronization mechanism in the USB drivers. So this leaves
> > ehci-s5p and phy-samsung-usb2 as the only possible places, and I chose
> > the latter since all the host-side HSIC initialization is also there
> > already. I think if you think of it as "reset whatever is on the other
> > side of this PHY", it's okay to put it as an optional feature into the
> > PHY driver.
> 
> CC'ed Tomasz Figa, Dongjin Kim, Yulgon Kim
> 
> 
> Hi Tomasz, Dongjin,
> 
> Julius Werner wants to put 'SMSC 3503 hub reset on Arndale board'
> to 'phy-samsung-usb*.c' files, because there is a timing dependency
> above mentioned.
> The following is the original patch sent by Julius Werner two day ago.
> (http://www.spinics.net/lists/linux-samsung-soc/msg20250.html)

Well, I think this is simply broken. Following are the reasons why I think so:
 a) you can use the smsc3503 chip on any USB/HSIC controller and with any 
USB/HSIC PHY, so you would have to add such reset GPIO to _every_ PHY or 
controller driver,
 b) you might want to use other USB/HSIC hubs like this one that requires some 
initialization sequence, other than just toggling a GPIO, so you would have to 
add cases for all of those hubs or other chips in _every_ PHY or controller 
driver.

> Previously, Olof mentioned that 'drivers/platform/arm/' would be used.
> (http://patches.linaro.org/16856/)
> 
> Also, another way was mentioned by Fabio Estevam, using
> 'drivers/reset/gpio-reset.c' which is not merged yet.
> (http://permalink.gmane.org/gmane.linux.drivers.devicetree/36830)
> 
> I think that 'phy-samsung-usb*.c' files are not a good place.
> However, Julius Werner's comment looks reasonable enough.

I can see this problem almost equivalent to the problem with on-board WLAN 
adapters connected using SDIO. Those adapters require some kind of power on 
sequence before they can be enumerated using standard MMC/SDIO mechanisms and 
so does this USB/HSIC hub.

So basically this is a thing that we should consider on a more generic level, 
not just for this particular single chip.

CCing some extra people to increase chance of getting more opinions on this.

Best regards,
Tomasz

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