Re: [PATCH] ARM: tegra: beaver: allow SD card voltage to be changed

[Adrian Hunter <adrian.hunter-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On 13/06/16 13:22, Jon Hunter wrote:
> Adding Adrian and Ulf ...
> 
> On 19/05/16 15:29, Jon Hunter wrote:
>>
>> On 13/05/16 18:27, Thierry Reding wrote:
>>> * PGP Signed by an unknown key
>>>
>>> On Fri, May 13, 2016 at 09:25:31AM +0200, Lucas Stach wrote:
>>>> Am Montag, den 29.02.2016, 22:01 +0100 schrieb Lucas Stach:
>>>>> This allows to switch the card signal voltage level to 1.8V,
>>>>> which is needed for any ultra high speed modes to work.
>>>>>
>>>>> Signed-off-by: Lucas Stach <dev-8ppwABl0HbeELgA04lAiVw@public.gmane.org>
>>>>> ---
>>>>> This needs the SDMMC memcomp pad calibration patches I just
>>>>> sent out to be applied, otherwise the card voltage change will
>>>>> fail with a message in the kernel log and a fall back to
>>>>> high speed operation.
>>>>
>>>> The patches this one depends on have been applied for some time now.
>>>> Please pick up this patch.
>>>
>>> My understanding is that UHS modes currently cause problems on Beaver.
>>> What I don't understand about that is how it will even try those modes
>>> if the voltage regulator can't be set to 1.8 V? Shouldn't that actively
>>> prevent those modes from even being attempted?
>>
>> Looking at the sdhci code, if the regulator is missing then we still
>> attempt to place the controller is 1.8V mode ...
>>
>>  static int sdhci_start_signal_voltage_switch(struct mmc_host *mmc,
>>                                               struct mmc_ios *ios)
>>  {
>>
>>  ...
>>
>>          case MMC_SIGNAL_VOLTAGE_180:
>>                  if (!IS_ERR(mmc->supply.vqmmc)) {
>>                          ret = regulator_set_voltage(mmc->supply.vqmmc,
>>                                          1700000, 1950000);
>>                          if (ret) {
>>                                  pr_warn("%s: Switching to 1.8V signalling voltage failed\n",
>>                                          mmc_hostname(mmc));
>>                                  return -EIO;
>>                          }
>>                  }
>>
>>                  /*
>>                   * Enable 1.8V Signal Enable in the Host Control2
>>                   * register
>>                   */
>>                  ctrl |= SDHCI_CTRL_VDD_180;
>>                  sdhci_writew(host, ctrl, SDHCI_HOST_CONTROL2);
>>  
>>                  /* Some controller need to do more when switching */
>>                  if (host->ops->voltage_switch)
>>                          host->ops->voltage_switch(host);
>>  
>>                  /* 1.8V regulator output should be stable within 5 ms */
>>                  ctrl = sdhci_readw(host, SDHCI_HOST_CONTROL2);
>>                  if (ctrl & SDHCI_CTRL_VDD_180)
>>                          return 0;
>>  
>>                  pr_warn("%s: 1.8V regulator output did not became stable\n",
>>                          mmc_hostname(mmc));
>>  
>>                  return -EAGAIN;
>>
>> Ideally, the above *should* fail if the regulator is missing. However, what
>> I have found, is that in my case, even though the regulator is missing, the
>> above succeeds and the host thinks we are operating at 1.8V even though we
>> are still at 3.3V! It seems that this does not happen with all SD cards that
>> support UHS. 
>>
>> This patch resolves the problems I am seeing on beaver with SD card
>> initialisation failing. I am surprised this is not causing problems for
>> others?
> 
> Adrian, Ulf, per the above, I have found that on a Tegra30 beaver board,
> if we enable UHS-I modes for Tegra30 but the device-tree for the board
> is missing the regulator to select 1.8V mode operation, then the above
> code sequence may still return success (ie. SDHCI_CTRL_VDD_180 bit is
> set in SDHCI_HOST_CONTROL2) even though we have not changed the voltage.
> This leads to other problems later on during SD initialisation.
> 
> Would you expect that an SDHCI controller should fail to set the
> SDHCI_CTRL_VDD_180 bit in the SDHCI_HOST_CONTROL2 register if we did not
> change the voltage?

What is meant to happen is that sdhci should wait 5ms and then check
SDHCI_CTRL_VDD_180 - which it used to do but then someone took the 5ms wait
away.

In any case, if you are using a regulator there is no knowing what sdhci is
meant to do.

> 
> We want to ensure that Tegra devices do not attempt to switch the UHS-I
> modes if the regulator is not present and it is not clear to me if there
> is a problem with the Tegra SDHCI controller or how this should be handled.

If the driver doesn't support UHS-I modes then it must remove the cap flags.