Date:	Mon, 9 Nov 2015 17:30:26 +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 05:00:46PM +0100, Ulf Hansson wrote:
> On 9 November 2015 at 15:40, Ludovic Desroches
> <ludovic.desroches@...el.com> wrote:
> > 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.
> >
> 
> Okay, I am finally starting to understand some of your concern.
> 
> According to the spec, the Power Control Register should control the
> signal voltage/bus voltage. As UHS mode was added to the spec, it
> seems like the Power Control Register couldn't cover all new cases, as
> why Host Control 2 register needed to be added. The Host Control 2
> register, is what sdhci_do_start_signal_voltage_switch() uses to
> change the signal voltage level, which all makes sense to me.
> 
> For sdhci_set_power(); it seems to use the Power Control Register to
> control the power to the card (VDD/VMMC). Indeed this looks *really*
> weird/wrong. I wonder if it's working because of luck, intentional
> violation of the SDHCI spec or because of special variants.
> 

Yes but I don't know if we are doing something weird/wrong or if it
is the naming of the 'SD Bus Voltage Select' which is weird/wrong.

> Especially when looking into the case when you *don't* have a VMMC
> regulator several strange quirks exists in sdhci_set_power().
> 
> In the case when you *have* a VMMC, I think just setting/clearing bit
> 0 (SDHCI_POWER_ON) and then bail out, is probably working with modern
> HW because it's likely the only thing needed.
> 
> Now, this discussion was interesting, but I forgot what problem you
> actually where trying to solve? :-)

There is this discussion because of two things:
- Fixing something I consider as a bug: when I have a VMMC, only
  setting/clearing bit 0. Our controller strictly obeys to the spec and
  check the 'SD Bus Voltage Select' field. Since we put a reserved value
  (000), the Power On is not performed.
- I was trying to get help to understand what is this 'SD Bus Voltage'.
  For our controller and sdhci_set_power(), it seems to stand for VMMC.
  For me, everything concerning bus voltage is related to VQMMC, so I was
  disappointed.


Regards

Ludovic
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