linux-kernel - Re: [PATCH 0/2] mmc: sdhci: potentially bad behavior when using vmmc supply

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Date:	Mon, 9 Nov 2015 11:25:55 +0100
From:	Ludovic Desroches <ludovic.desroches@...el.com>
To:	Ulf Hansson <ulf.hansson@...aro.org>
CC:	Nicolas Ferre <nicolas.ferre@...el.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	<pierre.samat@...el.com>,
	Ludovic Desroches <ludovic.desroches@...el.com>
Subject: Re: [PATCH 0/2] mmc: sdhci: potentially bad behavior when using vmmc
 supply

On Mon, Nov 09, 2015 at 10:38:03AM +0100, Ulf Hansson wrote:
> [...]
> 
> >> > Is the regulator-gpio usage the right thing to do for vqmmc? In my case it is
> >> > not really driven by a gpio but by a pio from the sdhci device. In the binding,
> >>
> >> What's a "pio"?
> >>
> >> What do you mean by the it's driven from the sdhci device?
> >>
> >
> > Sorry I mean sdhci device from the SoC point of view, I should say
> > controller. So yes the signal is driven by the controller.
> >
> >> Is it the internal HW logic of the sdhci controller that manages the
> >> IO voltage? And this logic can be controlled via certain register bits
> >> in the SDHCI controller?
> >>
> >
> > Yes, it depends of the value of the '1.8V Signaling Enable' value in the
> > host control 2 register.
> >
> >> > declaring the gpio is an option so I thought using this regulator fits my need.
> >>
> >> In quite many cases it makes sense to model this though a gpio
> >> regulator. For example when you use a level shifter circuit. Those
> >> normally have gpio pin routed to control the voltage level output for
> >> the signals. For example switching between 1.8V and 2.9V.
> >>
> >
> > I agree, my concern is to know if I can consider it as a 'general' pio
> > since it is driven by the sdhci controller.
> 
> This doesn't seems like a case where a gpio regulator should be used
> and I am not sure what problem it would solve. Beside to suppress the
> log warnings (actually those aren't warnings but informations).
> 
> Isn't sdhci_do_start_signal_voltage_switch() doing what you need here?
> 

It is. I am only wondering the best way to describe the hardware:
- No regulator but I have the 'no vqmmc regulator not found' message which
  is a bit annoying and which can be interpreted as an issue for someone
  who has no knowledge about this stuff.
- Describe the regulator since there is one on my board. But it is not a
  fixed regulator and even if it's close to a gpio one it is not.

Regards

Ludovic
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