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SubjectRe: [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@linaro.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@linaro.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@genesyslogic.com.tw>
> > >>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.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

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