Re: [PATCH V2 2/3] mmc: dw_mmc: Support voltage changes

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On 25 August 2014 22:59, Doug Anderson <dianders@xxxxxxxxxx> wrote:
> Ulf,
>
> On Mon, Aug 25, 2014 at 1:31 AM, Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
>> On 22 August 2014 22:38, Doug Anderson <dianders@xxxxxxxxxx> wrote:
>>> Ulf,
>>>
>>> On Fri, Aug 22, 2014 at 8:35 AM, Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
>>>> On 22 August 2014 15:47, Yuvaraj Kumar C D <yuvaraj.cd@xxxxxxxxx> wrote:
>>>>> From: Doug Anderson <dianders@xxxxxxxxxxxx>
>>>>>
>>>>> For UHS cards we need the ability to switch voltages from 3.3V to
>>>>> 1.8V.  Add support to the dw_mmc driver to handle this.  Note that
>>>>> dw_mmc needs a little bit of extra code since the interface needs a
>>>>> special bit programmed to the CMD register while CMD11 is progressing.
>>>>> This means adding a few extra states to the state machine to track.
>>>>>
>>>>> Signed-off-by: Doug Anderson <dianders@xxxxxxxxxxxx>
>>>>> Signed-off-by: Yuvaraj Kumar C D <yuvaraj.cd@xxxxxxxxxxx>
>>>>> ---
>>>>> changes since v1:
>>>>>         1. Added error message and return error in case of regulator_set_voltage() fail.
>>>>>         2. changed  dw_mci_cmd_interrupt(host,pending | SDMMC_INT_CMD_DONE)
>>>>>            to dw_mci_cmd_interrupt(host,pending).
>>>>>         3. Removed unnecessary comments.
>>>>>
>>>>>  drivers/mmc/host/dw_mmc.c  |  134 +++++++++++++++++++++++++++++++++++++++++---
>>>>>  drivers/mmc/host/dw_mmc.h  |    5 +-
>>>>>  include/linux/mmc/dw_mmc.h |    2 +
>>>>>  3 files changed, 131 insertions(+), 10 deletions(-)
>>>>>
>>>>> diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
>>>>> index aadb0d6..f20b4b8 100644
>>>>> --- a/drivers/mmc/host/dw_mmc.c
>>>>> +++ b/drivers/mmc/host/dw_mmc.c
>>>>> @@ -29,6 +29,7 @@
>>>>>  #include <linux/irq.h>
>>>>>  #include <linux/mmc/host.h>
>>>>>  #include <linux/mmc/mmc.h>
>>>>> +#include <linux/mmc/sd.h>
>>>>>  #include <linux/mmc/sdio.h>
>>>>>  #include <linux/mmc/dw_mmc.h>
>>>>>  #include <linux/bitops.h>
>>>>> @@ -234,10 +235,13 @@ err:
>>>>>  }
>>>>>  #endif /* defined(CONFIG_DEBUG_FS) */
>>>>>
>>>>> +static void mci_send_cmd(struct dw_mci_slot *slot, u32 cmd, u32 arg);
>>>>> +
>>>>>  static u32 dw_mci_prepare_command(struct mmc_host *mmc, struct mmc_command *cmd)
>>>>>  {
>>>>>         struct mmc_data *data;
>>>>>         struct dw_mci_slot *slot = mmc_priv(mmc);
>>>>> +       struct dw_mci *host = slot->host;
>>>>>         const struct dw_mci_drv_data *drv_data = slot->host->drv_data;
>>>>>         u32 cmdr;
>>>>>         cmd->error = -EINPROGRESS;
>>>>> @@ -253,6 +257,31 @@ static u32 dw_mci_prepare_command(struct mmc_host *mmc, struct mmc_command *cmd)
>>>>>         else if (cmd->opcode != MMC_SEND_STATUS && cmd->data)
>>>>>                 cmdr |= SDMMC_CMD_PRV_DAT_WAIT;
>>>>>
>>>>> +       if (cmd->opcode == SD_SWITCH_VOLTAGE) {
>>>>> +               u32 clk_en_a;
>>>>> +
>>>>> +               /* Special bit makes CMD11 not die */
>>>>> +               cmdr |= SDMMC_CMD_VOLT_SWITCH;
>>>>> +
>>>>> +               /* Change state to continue to handle CMD11 weirdness */
>>>>> +               WARN_ON(slot->host->state != STATE_SENDING_CMD);
>>>>> +               slot->host->state = STATE_SENDING_CMD11;
>>>>> +
>>>>> +               /*
>>>>> +                * We need to disable clock stop while doing voltage switch
>>>>> +                * according to Voltage Switch Normal Scenario.
>>>>> +                * It's assumed that by the next time the CLKENA is updated
>>>>> +                * (when we set the clock next) that the voltage change will
>>>>> +                * be over, so we don't bother setting any bits to synchronize
>>>>> +                * with dw_mci_setup_bus().
>>>>> +                */
>>>>
>>>> I don't know the details about the dw_mmc controller, but normally a
>>>> host driver is expected to gate the clock from it's ->set_ios
>>>> callback, when the clk frequency are set to 0.
>>>>
>>>> Could you elaborate on why dw_mmc can't do that, but need to handle
>>>> this from here?
>>>
>>> Let's see, it's been a while since I wrote this...
>>>
>>>
>>> So dw_mmc has a special feature where the controller itself will
>>> automatically stop the MMC Card clock when nothing is going on.  This
>>> is called "low power" mode.  I'm not super familiar with other MMC
>>> drivers, I get the impression that this is a special dw_mmc feature
>>> and not common to other controllers.  I think other drivers have
>>> aggressive runtime PM to get similar power savings.
>>
>> I see.
>>
>> I am familiar with such "low power" mode features, there are certainly
>> other controllers supporting such as well. My experience tells me,
>> it's hard to get things right for all corner cases. The voltage switch
>> behaviour is just one of these, then you have SDIO irq etc.
>>
>> Instead of using the controller HW, yes you may implement clock gating
>> through runtime PM in the host driver.
>>
>>>
>>> The dw_mmc auto clock gating can wreck total havoc on the voltage
>>> change procedure, since part of the way that the host and card
>>> communicate is that the host is supposed to stop its clock when it
>>> gets to a certain phase of the voltage switch sequence.  If the
>>> controller is stopping the clock for us then it can confuse the card.
>>>
>>> The dw_mmc manual says that before starting a voltage change you must
>>> turn off low power mode.  That's what we're doing here.
>>>
>>>
>>> The comment above refers to the fact dw_mci_setup_bus() will
>>> unconditionally turn low power mode back on for us when called with a
>>> non-zero clock.  If dw_mci_setup_bus() might be called with a non-zero
>>> clock during the voltage change then this would be disaster (low power
>>> mode would be back on!).  ...but the clock will always be zero during
>>> the voltage change and when it's finally non-zero then it's the last
>>> stage of the voltage change and we can go back to low power mode.
>>>
>>>
>>> Possibly the above was not super clear from the comment (even for
>>> those familiar with the oddities of dw_mmc).  If you want me to try to
>>> rewrite the comment, let me know.
>>
>> I appreciate an updated comment, it's nice to know what goes on. :-)
>
> OK, how about:
>
> /*
>  * We need to disable low power mode (automatic clock stop)
>  * while doing voltage switch so we don't confuse the card,
>  * since stopping the clock is a specific part of the UHS
>  * voltage change dance.
>  *
>  * Note that low power mode (SDMMC_CLKEN_LOW_PWR) will be
>  * unconditionally turned back on in dw_mci_setup_bus() if it's
>  * ever called with a non-zero clock.  That shouldn't happen
>  * until the voltage change is all done.
>  */
>
> Yuvaraj: can you include that in the next patch set you send out?

Thanks! Applied for next!

I took the liberty to change to the clarified comment from Doug above.

Kind regards
Uffe
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