Re: [PATCH V1] mmc: core: Wait for 1ms after enabling the clocks post voltage switch

[Adrian Hunter]

On 17/11/22 13:07, Ulf Hansson wrote:
> On Thu, 17 Nov 2022 at 10:49, Sarthak Garg <quic_sartgarg@xxxxxxxxxxx> wrote:
>>
>> As per spec we should wait for 1ms after providing the SD clock to the
>> card again as part of voltage switch sequence but there seems to be a
>> violation here. Clocks are getting gated before 1ms as part of
>> sdhci_set_ios function where we try to reset SD Clock Enable by
>> resetting SDHCI_CLOCK_CARD_EN bit in SDHCI_CLOCK_CONTROL register
>> leading to voltage switch failures for specific SD cards.
>> Below ftraces also confirms the above understanding :
>>
>> 9.511367: mmc_host_set_uhs_voltage: mmc1 called
>> 9.511369: mmc_set_ios: mmc1: clock 0Hz busmode 2 powermode 2 cs 0
>> Vdd 18 width 1 timing 0
>> 9.511370: sdhci_set_ios: mmc1 called
>> 9.511370: sdhci_set_ios: mmc1 setting clock ios->clock: 0 host->clock:
>> 400000
>> 9.511372: sdhci_msm_set_clock: mmc1 clock: 0
>> 9.511394: sdhci_set_ios: mmc1 gating clocks by writing
>> ~SDHCI_CLOCK_CARD_EN to SDHCI_CLOCK_CONTROL register
>> 9.511413: sdhci_msm_set_clock: mmc1 clock: 0
>> 9.511423: mmc_set_signal_voltage: mmc1 called
>> 9.533816: mmc_set_ios: mmc1: clock 400000Hz busmode 2 powermode 2 cs 0
>> Vdd 18 width 1 timing 0
>> 9.533820: sdhci_set_ios: mmc1 called
>> 9.533822: sdhci_set_ios: mmc1 setting clock ios->clock: 400000
>> host->clock: 0
>> 9.533826: sdhci_msm_set_clock: mmc1 clock: 400000
>> 9.533925: sdhci_enable_clk: mmc1 Enabling clocks by writing
>> SDHCI_CLOCK_CARD_EN to SDHCI_CLOCK_CONTROL register
>> 9.533950: sdhci_set_ios: mmc1 gating clocks by writing
>> ~SDHCI_CLOCK_CARD_EN to SDHCI_CLOCK_CONTROL register
>> 9.533973: sdhci_msm_set_clock: mmc1 clock: 400000
>> 9.534043: sdhci_enable_clk: mmc1 Enabling clocks by writing
>> SDHCI_CLOCK_CARD_EN to SDHCI_CLOCK_CONTROL register
>> 9.534045: mmc_host_set_uhs_voltage: mmc1 Done
>>
>> Wait for 1ms after enabling the clock post voltage switch to make sure
>> clock is kept alive for alteast 1ms as per spec.
>>
>> Signed-off-by: Sarthak Garg <quic_sartgarg@xxxxxxxxxxx>
> 
> I don't know the exact behaviour of sdhci around this, so I will defer
> to Adrian's input for that.

sdhci_set_ios() seems to be mucking around with the clock way
more than necessary.  I'll go over it and see what can be done.

> 
> However, let me point out that in mmc_set_uhs_voltage() we are trying
> to take the 1ms into account. More precisely, after
> mmc_host_set_uhs_voltage() has been called to switch the voltage to
> 1.8V and to re-enable the clock, mmc_set_uhs_voltage() does a
> "mmc_delay(1)" and then it calls the ->card_busy() ops to check that
> card doesn't signal busy by holding DAT0 low.
> 
> I understand that the code in mmc_set_uhs_voltage(), expects the host
> to be rather dumb from the HW perspective, which may not always be the
> case. Although, I would rather avoid introducing new host flags, along
> what you propose in the $subject patch. If this can't be managed in
> sdhci, without some new help from the mmc core, I would rather suggest
> that we introduce a new host callback that can be used to replace the
> entire part in mmc_host_set_uhs_voltage() (or something along those
> lines).
> 
> Kind regards
> Uffe
> 
>> ---
>>  drivers/mmc/core/core.c  | 4 ++++
>>  drivers/mmc/host/sdhci.c | 3 +++
>>  include/linux/mmc/host.h | 1 +
>>  3 files changed, 8 insertions(+)
>>
>> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
>> index a1efda85c6f2..d63e00aab6cb 100644
>> --- a/drivers/mmc/core/core.c
>> +++ b/drivers/mmc/core/core.c
>> @@ -1181,6 +1181,8 @@ int mmc_host_set_uhs_voltage(struct mmc_host *host)
>>         host->ios.clock = 0;
>>         mmc_set_ios(host);
>>
>> +       host->doing_signal_voltage_switch = true;
>> +
>>         if (mmc_set_signal_voltage(host, MMC_SIGNAL_VOLTAGE_180))
>>                 return -EAGAIN;
>>
>> @@ -1189,6 +1191,8 @@ int mmc_host_set_uhs_voltage(struct mmc_host *host)
>>         host->ios.clock = clock;
>>         mmc_set_ios(host);
>>
>> +       host->doing_signal_voltage_switch = false;
>> +
>>         return 0;
>>  }
>>
>> diff --git a/drivers/mmc/host/sdhci.c b/drivers/mmc/host/sdhci.c
>> index fb6e9a81f198..ac7c254eef4b 100644
>> --- a/drivers/mmc/host/sdhci.c
>> +++ b/drivers/mmc/host/sdhci.c
>> @@ -2312,6 +2312,9 @@ void sdhci_set_ios(struct mmc_host *mmc, struct mmc_ios *ios)
>>                 host->ops->set_clock(host, ios->clock);
>>                 host->clock = ios->clock;
>>
>> +               if (mmc->doing_signal_voltage_switch)
>> +                       usleep_range(1000, 1250);
>> +
>>                 if (host->quirks & SDHCI_QUIRK_DATA_TIMEOUT_USES_SDCLK &&
>>                     host->clock) {
>>                         host->timeout_clk = mmc->actual_clock ?
>> diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
>> index 8fdd3cf971a3..06c88cd7a8bf 100644
>> --- a/include/linux/mmc/host.h
>> +++ b/include/linux/mmc/host.h
>> @@ -521,6 +521,7 @@ struct mmc_host {
>>         bool                    hsq_enabled;
>>
>>         u32                     err_stats[MMC_ERR_MAX];
>> +       bool                    doing_signal_voltage_switch;
>>         unsigned long           private[] ____cacheline_aligned;
>>  };
>>
>> --
>> 2.17.1
>>