Hi Subhash,
2012/12/8 Subhash Jadavani <subha...@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?
I'm developing for the MMCI driver, but the UHS support code is still
pending mainlining.
Kind regards, Johan
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