linux-kernel - Re: [RFC PATCH V3 15/21] mmc: sdhci: UHS-II support, modify set

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Message-ID: <20200915062443.GB2860208@laputa>
Date:   Tue, 15 Sep 2020 15:24:43 +0900
From:   AKASHI Takahiro <takahiro.akashi@...aro.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Ben Chuang <benchuanggli@...il.com>, ulf.hansson@...aro.org,
        linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org,
        ben.chuang@...esyslogic.com.tw, greg.tu@...esyslogic.com.tw
Subject: Re: [RFC PATCH V3 15/21] mmc: sdhci: UHS-II support, modify
 set_power() to handle vdd2

Adrain,

On Mon, Sep 14, 2020 at 09:36:02AM +0300, Adrian Hunter wrote:
> On 14/09/20 8:45 am, AKASHI Takahiro wrote:
> > Adrian,
> > 
> > On Fri, Aug 21, 2020 at 05:11:18PM +0300, Adrian Hunter wrote:
> >> On 10/07/20 2:11 pm, Ben Chuang wrote:
> >>> From: AKASHI Takahiro <takahiro.akashi@...aro.org>
> >>>
> >>> VDD2 is used for powering UHS-II interface.
> >>> Modify sdhci_set_power_and_bus_voltage(), sdhci_set_power_noreg()
> >>> and sdhci_set_power_noreg() to handle VDD2.
> >>
> >> vdd2 is always 1.8 V and I suspect there may never be support for anything
> >> else, so we should start with 1.8 V only.
> > 
> > What do you mean here?
> > You don't want to add an extra argument, vdd2, to sdhci_set_power().
> > Correct?
> 
> Yes
> 
> > 
> >> Also can we create uhs2_set_power_reg() and uhs2_set_power_noreg() and use
> >> the existing ->set_power() callback
> > 
> > Again what do you expect here?
> > 
> > Do you want to see any platform-specific mmc driver who supports UHS-II
> > to implement its own call back like:
> 
> Not exactly.  I expect there to be a common implementation in sdhci-uhs2.c
> called sdhci_uhs2_set_power() for example, that drivers can use by setting
> their .set_power = sdhci_uhs2_set_power.  If they need platform-specific
> code as well then their platform-specific code can call
> sdhci_uhs2_set_power() if desired.
> 
> > 
> > void sdhci_foo_set_power(struct sdhci_host *host, unsigned char mode,
> >                                   unsigned short vdd)
> > {
> >         sdhci_set_power(host, mode,vdd);
> > 
> >         /* in case that sdhci_uhs2 module is not inserted */
> >         if (!(mmc->caps & MMC_CAP_UHS2))
> >                 return;
> > 
> >         /* vdd2 specific operation */
> >         if (IS_ERR_OR_NULL(host->mmc->supply.vmmc2))
> >                 sdhci_uhs2_set_power_noreg(host, mode);
> >         else
> >                 sdhci_uhs2_set_power_reg(host, mode);
> > 
> >         /* maybe more platform-specific initialization */
> > }
> > 
> > struct sdhci_ops sdhci_foo_ops = {
> >         .set_power = sdhci_foo_set_power,
> >         ...
> > }

What do you think about this logic in general?
(If necessary, read it replacing "foo" to "uhs2".)

What I'm concerned about is SDHCI_POWER_CONTROL register.
Vdd and vdd2 are controlled with corresponding bits in this register.
It seems to be "natural" to me that vdd and vdd2 are enabled
in a single function rather than putting them in separate ones.

In particular, in the case of sdhci_set_power_noreg(), there exist a couple
of "quirks" around writing the bits to SDHCI_POWER_CONTROL register.
I don't know how we should handle them if we have a separate function,
say, sdhci_uhs2_set_power_noreg().
Do you want to see a copy of the same logic in sdhci_uhs2_set_power_noreg()? 

-Takahiro Akashi


> > 
> > Is this what you mean?
> > (I'm not quite sure yet that sdhci_ush2_set_power_noreg() can be split off
> > from sdhci_set_power_noreg().)
> > 
> > -Takahiro Akashi