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

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Date:   Tue, 15 Sep 2020 15:03:06 +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()

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.

I have been confused with this comment.
(I know it came from the original Intel code, not from Ben.)

If this comment is true,

> >>>  	 */
> >>> -	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))

the condition above must be wrong since 'flags & MMC_UHS2_SUPPORT'
is one of capabilities for a host controller, not a card
while the selection of voltage depends on a card type.

So I wonder why this code still works.
I guess that it is because set_signal_voltage(), or other variant functions,
will never be called for UHS-II cards under the current implementation.

Looking at mmc_sd_init_card(), we have added some hack:
mmc_sd_init_card()
{
        ...
        /* For UHS2, skip the UHS-I initialization. */
        if ((host->flags & MMC_UHS2_SUPPORT) &&
            (host->flags & MMC_UHS2_INITIALIZED))
                goto done;
        ...
                if (mmc_sd_card_using_v18(card)) {
                        if (mmc_host_set_uhs_voltage(host) ||
                            mmc_sd_init_uhs_card(card)) {
                ...
}

Ben, can you confirm this?
(There is another callsite of mmc_host_set_uhs_voltage() though.)

> >> 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.

-Takahiro Akashi