Re: [PATCH 3/3] mmc: core: Fixup signal voltage switch

[Subhash Jadavani <subhashj@codeaurora.org>]

On 12/10/2012 1:51 PM, Johan Rudholm wrote:
> Hi Subhash,
>
> 2012/12/8 Subhash Jadavani <subhashj@codeaurora.org>:
>> On 12/7/2012 9:49 PM, Johan Rudholm wrote:
>>> When switching SD and SDIO cards from 3.3V to 1.8V signal levels, the
>>> clock should be gated for 5 ms during the step. After enabling the
>>> clock, the host should wait for at least 1 ms before checking for
>>> failure. Failure by the card to switch is indicated by dat[0:3] being
>>> pulled low. The host should check for this condition and power-cycle
>>> the card if failure is indicated.
>>>
>>> Add a retry mechanism for the SDIO case.
>>>
>>> If the voltage switch fails repeatedly, give up and continue the
>>> initialization using the original voltage.
>>>
>>> This patch places a couple of requirements on the host driver:
>>>
>>>    1) mmc_set_ios with ios.clock = 0 must gate the clock
>>>    2) mmc_power_off must actually cut the power to the card
>>>
>>> if these requirements are not fulfilled, the 1.8V signal voltage switch
>>> may not be successful.
> <snip>
>
>>>                  err = host->ops->start_signal_voltage_switch(host,
>>> &host->ios);
>>
>> Shouldn't you fix all the existing host drivers who have already implemented
>> start_signal_voltage_switch host ops? If you don't change them as part of
>> this patch then
>> i afraid UHS functionality would be broken on those platforms. Also, it's
>> not just changing the start_signal_voltage_switch hot op implementation, you
>> may also need to add card_busy() host op implementation for those drivers.
> This is true, and I did actually make an RFC for the sdhci driver
> ("[RFC] mmc: sdhci: Let core handle UHS switch failure":
> https://patchwork.kernel.org/patch/1517211/). Daniel Drake tried this
> code for a problem related to the 1.8V switch (his board could
> actually not perform the switch to 1.8V even though this capability
> was announced), and I think this pointed out two areas for further
> investigation before a proper patch for the sdhci driver can be
> created:
>
> 1) the sdhci driver may not gate the clock when setting ios.clock = 0
> and calling mmc_set_ios
> 2) mmc_power_off may not cut power to the card
>
> maybe 2) was only for Daniel's board, I'm not sure, but this needs to
> be investigated further anyway. Since I don't have any sdhci hardware
> with UHS support, this must either be done by some other kind soul or
> it will have to wait until I get the hardware.
>
> sdhci is the only driver I'm aware of that's got (mainlined) support
> for start_signal_voltage_switch, are you thinking of any other driver?
You may look at only in tree drivers who have implemented 
start_singnal_voltage_switch() ops. Our msm_sdcc* driver has also 
implemented it but it's not in tree driver so i may
need to take care of that later once we are synced to kernel version 
which will have your patch series.

Yes, SDHCi seeems to be only one in tree driver implemented the 
start_singnal_voltage_switch() ops. So you might be good fixing the same.
> I'm developing for the MMCI driver, but the UHS support code is still
> pending mainlining.
>
> Kind regards, Johan