linux-kernel - Re: [RFC PATCH v3] reset: Add a defer reset object to send board specific reset

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Date:	Thu, 14 Aug 2014 11:36:38 +0200
From:	Philipp Zabel <p.zabel@...gutronix.de>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	Houcheng Lin <houcheng@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Stephen Gallimore <stephen.gallimore@...com>,
	Srinivas KANDAGATLA <srinivas.kandagatla@...com>,
	Ulf Hansson <ulf.hansson@...aro.org>, sre@...nel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Vikas Sajjan <vikas.sajjan@...sung.com>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH v3] reset: Add a defer reset object to send board
 specific reset

Hi Maxime,

Am Montag, den 11.08.2014, 19:33 +0200 schrieb Maxime Ripard:
> > Mostly because Maxime and I seem to have a completely different opinion
> > and nobody else argued one way or the other.
> 
> Yep, mostly because I don't see how a generic approach can work.
> 
> The existing reset-gpios property only provide the gpio to use, but
> some informations are encoded in the driver, such as the reset
> duration, or a reset sequence if any.

The driver should provide the duration. I'd really like to see an
example where sequencing is necessary.
I agree that as soon as things get significantly more complicated than
pulsing a single GPIO, the reset-gpios binding is too limited.
Still, I'm not happy to mandate a separate gpio reset device for each
reset line if most devices are simple enough for it to work without.
What about using reset-gpios for the majority of simple cases and have a
separate gpio-reset-sequencer driver when multiple GPIO resets have to
be timed?

> How do you plan on giving that information to your generic driver?
> 
> The only solution I can think of would be to add an extra property
> that your code would parse. But then, you break the existing DT
> bindings.
>
> And if we're going to break those bindings, at least do it in a way
> consistent with reset bindings.

For the backwards compatibility case, the driver already has to provide
the duration. I don't want to break the existing bindings at all.

> Plus, your approach doesn't cover the weird corner cases such as:
>   - reset-gpio
>   - wlf,reset-gpios
>   - phy-reset-gpios
>   - snps,reset-gpio
>   - the drivers that need several gpio and expect the reset one as a
>     positional argument.
>   - etc.

Those are just an issue of the implementation I posted earlier because
gpiod_get doesn't support custom names other than %s-gpios. This could
be extended and handled just as well if deemed necessary.

regards
Philipp

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