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

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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?
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