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

[Jon Hunter <jonathanh-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>]

On 14/06/16 07:20, Adrian Hunter wrote:
> 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.

Do you plan to add the 5ms delay again?

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

Ok, seems fragile.

>>
>> 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.

So the controller itself supports UHS-I modes, but a given board may not
have the regulator to support them. We need a way to determine if the
board can support the UHS-I modes. Now we could check to see if the
regulator is present in the Tegra SDHCI driver and if not remove the cap
flags. However, I was not sure if this is applicable to other sdhci
controllers and so there should be a generic solution for this?

Cheers
Jon

-- 
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