linux-kernel - Re: [PATCH 1/2] mmc: sdhci: set bus voltage before enabling bus power

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Message-ID: <20151109144030.GD15706@odux.rfo.atmel.com>
Date:	Mon, 9 Nov 2015 15:40:30 +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 1/2] mmc: sdhci: set bus voltage before enabling bus power

On Mon, Nov 09, 2015 at 03:12:46PM +0100, Ulf Hansson wrote:
> On 9 November 2015 at 14:23, Ludovic Desroches
> <ludovic.desroches@...el.com> wrote:
> > On Fri, Nov 06, 2015 at 04:59:29PM +0100, Ludovic Desroches wrote:
> >> When there is a vmmc regulator, only SD Bus Power is set to 1 in the
> >> Power Control Register. It means SD Bus Voltage Select field is set to 0
> >> that is a reserved value. The SD Host Controller specification says:
> >> 'SD Bus Power: Before setting this bit, the SD Host Driver shall set SD
> >> Bus Voltage Select.' and 'If the Host Driver selects an unsupported
> >> voltage in the SD B?us Voltage Select field, the Host Controller may
> >> ignore writes to SD Bus Power and keep its value at zero."
> >>
> >> Having an external regulator means the SD Bus Voltage Select is useless
> >> but if the Host Controller strictly follows the specification then we
> >> need to set a valid value.
> >
> > Ulf,
> >
> > What is your opinion about this patch?
> >
> > If the 'no regulator found' message is turned in debug message, I can get
> > rid of my vmmc regulator but I really think that writing only
> 
> I expect you mean vqmmc?

I don't mean vmmc. In the sdhci_set_power function, we are using vmmc.
I feel not confortable with it because the power control register
contains 'SD Bus' fields so it should depend on vqmmc not vmmc.

> 
> > SDHCI_POWER_ON is opposite to the sdhci spec. I would say that not
> > setting the bus voltage is a quirk!
> 
> I don't really follow.
> 
> I read the SDHCI spec and the section for the Power Control Register.
> Bit 0 needs to be set when communicating with the card as it will for
> example enables the clock. Before setting bit0 you must decide what
> signal level to use, which is done by writing to bit 1->3.
> 

Right. But when having vmmc supply we do:
sdhci_writeb(host, SDHCI_POWER_ON, SDHCI_POWER_CONTROL) or
sdhci_writeb(host, 0, SDHCI_POWER_CONTROL) so we loose the signal level,
isn't it? 

> If SDHCI monitors the power state (MMC_POWER_UP|ON|OFF) when its
> ->set_ios() callback are invoked and in combination of using the
> ->start_signal_voltage_switch() callback to change the signal voltage
> level, this *should* work out nicely.
> 

It is my turn to not follow! We write into the Power Control Register
only in sdhci_set_power(). May I miss a callback or something else?
sdhci_do_start_signal_voltage_switch doesn't modify the Power Control
Register.

> Now, looking at the related code in sdhci, I am kind of surprised that
> it works. :-) Though, again I don't have the in-depth knowledge about
> sdhci.
> 

Me too, I am starting to dig into the sdhci spec and some points are
not crystal clear.


Regards

Ludovic
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