linux-kernel - Re: [RFC v2 00/13] usb/mmc/power: Fix USB/LAN when TFTP booting

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Date:	Tue, 31 May 2016 08:58:08 +0800
From:	Peter Chen <hzpeterchen@...il.com>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	linux-usb@...r.kernel.org, Sebastian Reichel <sre@...nel.org>,
	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mark Brown <broonie@...nel.org>, hverkuil@...all.nl,
	tjakobi@...h.uni-bielefeld.de,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC v2 00/13] usb/mmc/power: Fix USB/LAN when TFTP booting

On Sat, May 28, 2016 at 11:36:13AM +0800, Peter Chen wrote:
> On Tue, May 10, 2016 at 01:02:08PM +0200, Ulf Hansson wrote:
> > + Arnd
> > 
> > [...]
> > 
> > >> >> Solution
> > >> >> ========
> > >> >> This is very similar to the MMC pwrseq behavior so the idea is to:
> > >> >> 1. Move MMC pwrseq drivers to generic place,
> > >> >
> > >> > You can do that, but I'm going to NAK any use of pwrseq bindings outside
> > >> > of MMC. I think it is the wrong way to do things. The DT should describe
> > >>
> > >> Huh, I didn't know that was your view of the mmc pwrseq bindings. Why
> > >> didn't you NAK them before?
> > >
> > > Unfortunately, either I missed it or it was a time I couldn't spend much
> > > time on reviews.
> > 
> > Okay, I guess it's common issue among maintainers. The problem with DT
> > is that it gets really hard to be fixed up later. :-)
> > 
> > >
> > >> > the devices. If they happen to be "simple" then the core can walk the
> > >> > tree and do any setup. For example, look for "reset-gpios" and toggle
> > >> > that GPIO. There is no need for a special node.
> > >> >
> > >> >> 2. Extend the pwrseq-simple with regulator toggling,
> > >> >> 3. Add support to USB hub and port core for pwrseq,
> > >> >
> > >> > We discussed this for USB already[1] and is why we defined how to add
> > >> > USB child devices. The idea is not to add pwrseq to that.
> > >>
> > >> I am not familiar with the USB discussion.
> > >>
> > >> Still, let me give you some more background to the mmc pwrseq. The
> > >> idea from the mmc pwrseq bindings comes from the power-domain DT
> > >> bindings, as I thought these things were a bit related.
> > >> In both cases they are not directly a property of the device, but more
> > >> describing a HW dependency to allow the device to work.
> > >
> > > I could see this as a board level power domain. However the difference
> > > is we are not generally exposing internal SOC details the same way as
> > > board level components. Perhaps we could extend power domains to board
> > > level, but that is not what was done here.
> > >
> > >> One could probably use a child node instead of a phandle, but that
> > >> wasn't chosen back then. Of course you are the DT expert, but could
> > >> you perhaps tell me why a child node is better for cases like this?
> > >
> > > If there is a control path hierarchy, then we try to model that in DT
> > > with child nodes. In cases of SDIO and USB, there is a clear hierarchy.
> > > Ignoring the discovery ordering problem, we already have defined ways to
> > > describe GPIO connections, regulators, etc. to devices. Describing those
> > > things separately from the device to solve a particular issue that is
> > > really a kernel limitation is what I don't like.
> > 
> > Okay, I see.
> > 
> > To move forward in trying to make mmc pwrseq a generic pwrseq, could
> > we perhaps allow both cases?
> > 
> > In the mmc case, there are already deployed bindings so we need to
> > cope with these by using the phandle option, but for USB etc we could
> > force the child node option.
> > As long as we agree that we keep using a compatible string for the
> > child node as well, both options should be able to co-exist and we
> > should probably be able to managed them both from a common pwrseq
> > driver framework.
> > 
> > Although, I do remember from an older conversations around some of
> > mine submission for the mmc pwrseq code, that some people (maybe
> > Arnd?) wasn't keen on adding a new framework for this. Perhaps that
> > has changed?
> > 
> 
> All, how we move on for this?
> 
> 1. Using a generic driver to manage both mmc and USB (and further
> subsystem), USB and further subsystem do not use pwrseq node in dts.
> 2. USB creates the similar driver under drivers/usb for its own use. 
> 
> Which one do you prefer, thanks.
> 

Hi Krzysztof Kozlowski,

I think option 1 may be better according to Rob and Ulf's comment.
Would you like going on your patch set? You can work on generic pwrseq
driver, I will do USB stuffs based on generic pwrseq driver?

-- 

Best Regards,
Peter Chen