linux-kernel - Re: [RFC PATCH V3 13/21] mmc: sdhci: UHS-II support, skip signal_voltage

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Date:   Wed, 16 Sep 2020 15:24:12 +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 13/21] mmc: sdhci: UHS-II support, skip
 signal_voltage_switch()

On Wed, Sep 16, 2020 at 09:01:20AM +0300, Adrian Hunter wrote:
> On 15/09/20 9:03 am, AKASHI Takahiro wrote:
> > Ben, Adrian,
> > 
> > On Mon, Sep 14, 2020 at 11:08:14AM +0300, Adrian Hunter wrote:
> >> On 14/09/20 9:40 am, AKASHI Takahiro wrote:
> >>> Adrian,
> >>>
> >>> On Fri, Aug 21, 2020 at 05:09:01PM +0300, Adrian Hunter wrote:
> >>>> On 10/07/20 2:11 pm, Ben Chuang wrote:
> >>>>> From: AKASHI Takahiro <takahiro.akashi@...aro.org>
> >>>>>
> >>>>> sdhci_start_signal_voltage_switch() should be called only in UHS-I mode,
> >>>>> and not for UHS-II mode.
> >>>>>
> >>>>> Signed-off-by: Ben Chuang <ben.chuang@...esyslogic.com.tw>
> >>>>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@...aro.org>
> >>>>> ---
> >>>>>  drivers/mmc/host/sdhci.c | 7 ++++++-
> >>>>>  1 file changed, 6 insertions(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/drivers/mmc/host/sdhci.c b/drivers/mmc/host/sdhci.c
> >>>>> index 5511649946b9..7f2537648a08 100644
> >>>>> --- a/drivers/mmc/host/sdhci.c
> >>>>> +++ b/drivers/mmc/host/sdhci.c
> >>>>> @@ -2623,8 +2623,13 @@ int sdhci_start_signal_voltage_switch(struct mmc_host *mmc,
> >>>>>  	/*
> >>>>>  	 * Signal Voltage Switching is only applicable for Host Controllers
> >>>>>  	 * v3.00 and above.
> >>>>> +	 * But for UHS2, the signal voltage is supplied by vdd2 which is
> >>>>> +	 * already 1.8v so no voltage switch required.
> >>>>>  	 */
> >>>>> -	if (host->version < SDHCI_SPEC_300)
> >>>>> +	if (host->version < SDHCI_SPEC_300 ||
> >>>>> +	    (IS_ENABLED(CONFIG_MMC_SDHCI_UHS2) &&
> >>>>> +	     host->version >= SDHCI_SPEC_400 &&
> >>>>> +	     host->mmc->flags & MMC_UHS2_SUPPORT))
> >>>> Please look at hooking ->start_signal_voltage_switch() instead
> >>>
> >>> Do you mean that you want every platform driver who wants to support UHS-II
> >>> to set NULL to start_signal_voltage_switch hook even if this hack is
> >>> platform agnostic?
> >>
> >> No, I see UHS-II as a separate layer i.e.
> >>
> >>  UHS-II host controller driver
> >>   |   |
> >>   |   v
> >>   |   sdhci-uhs2 e.g. sdhci_uhs2_start_signal_voltage_switch
> >>   |   |
> >>   v   v
> >>   sdhci e.g. sdhci_start_signal_voltage_switch
> >>
> >> Most things should go through sdhci-uhs2 but not nessarily everything.
> > 
> > What I meant by my previous comment is that we don't have to
> > call any function, sdhci_uhs2_start_signal_voltage_switch in above example,
> > for UHS-II cards in any case since it is always simply empty.
> 
> Please treat the sdhci_uhs2_... host ops as functions for a UHS-II host
> controller in either UHS-II or legacy mode. i.e. it is up to sdhci-uhs2.c to
> call through to sdhci.c not the other way around.  e.g.
>
> int sdhci_uhs2_start_signal_voltage_switch(blah)
> {
> 	if (sdhci_uhs2_mode(host))
> 		return 0;
> 	return sdhci_start_signal_valtage_switch(blah);
> }

Okay, the remaining issue is to clarify the meaning of MMC_UHS2_SUPPORT.

-Takhiro Akashi