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 19:36:14 +0800
From:   Ben Chuang <benchuanggli@...il.com>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Ben Chuang <benchuanggli@...il.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ben Chuang <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()

Hi Takahiro,

On Tue, Sep 15, 2020 at 2:03 PM AKASHI Takahiro
<takahiro.akashi@...aro.org> 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.
>
> 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.

The flag MMC_UHS2_SUPPORT is set at the beginning of mmc_uhs2_rescan_try_freq().
In UHS-II flow, it stays set.
If the attempt to UHS-II fails finally, it will be unset.

>
> 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.)

UHS-II cards use differential signals and don't need to signal voltage switch.
But the main task is to set the parameters of UHS-II card interface.

>
> > >> 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