[PATCH 4/4] mmc: core: hold SD Clock before CMD11 during Signal

[Vincent.Yang]

>-----Original Message-----
>From: Russell King - ARM Linux [mailto:linux at arm.linux.org.uk]
>On Sun, Jul 13, 2014 at 01:21:10PM +0800, Vincent Yang wrote:
>> Voltage Switch Procedure
>>
>> This patch is to fix an issue found on mb86s7x platforms.
>>
>> [symptom]
>> There are some UHS-1 SD memory cards sometimes cannot be detected correctly,
>> e.g., Transcend 600x SDXC 64GB UHS-1 memory card.
>> During Signal Voltage Switch Procedure, failure to switch is indicated
>> by the card holding DAT[3:0] low.
>>
>> [analysis]
>> According to SD Host Controller Simplified Specification Version 3.00
>> chapter 3.6.1, the Signal Voltage Switch Procedure should be:
>> (1) Check S18A; (2) Issue CMD11; (3) Check CMD 11 response;
>> (4) Stop providing SD clock; (5) Check DAT[3:0] should be 0000b;
>> (6) Set 1.8V Signal Enable; (7) Wait 5ms; (8) Check 1.8V Signal Enable;
>> (9) Provide SD Clock; (10) Wait 1ms; (11) Check DAT[3:0] should be 1111b;
>> (12) error handling
>>
>> With CONFIG_MMC_CLKGATE=y, sometimes there is one more gating/un-gating
>> SD clock between (2) and (3). In this case, some UHS-1 SD cards will hold
>> DAT[3:0] 0000b at (11) and thus fails Signal Voltage Switch Procedure.
>>
>> [solution]
>> By mmc_host_clk_hold() before CMD11, the additional gating/un-gating
>> SD clock between (2) and (3) can be prevented and thus no failure at (11).
>> It has been verified with many UHS-1 SD cards on mb86s7x platforms and
>> works correctly.
>>
>> Signed-off-by: Vincent Yang <Vincent.Yang at tw.fujitsu.com>
>> ---
>>  drivers/mmc/core/core.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
>> index 7dc0c85..764af63 100644
>> --- a/drivers/mmc/core/core.c
>> +++ b/drivers/mmc/core/core.c
>> @@ -1428,6 +1428,8 @@ int mmc_set_signal_voltage(struct mmc_host *host, int
>signal_voltage, u32 ocr)
>>               pr_warning("%s: cannot verify signal voltage switch\n",
>>                               mmc_hostname(host));
>>
>> +     mmc_host_clk_hold(host);
>> +
>>       cmd.opcode = SD_SWITCH_VOLTAGE;
>>       cmd.arg = 0;
>>       cmd.flags = MMC_RSP_R1 | MMC_CMD_AC;
>> @@ -1438,8 +1440,6 @@ int mmc_set_signal_voltage(struct mmc_host *host, int
>signal_voltage, u32 ocr)
>>
>>       if (!mmc_host_is_spi(host) && (cmd.resp[0] & R1_ERROR))
>>               return -EIO;
>> -
>> -     mmc_host_clk_hold(host);
>
>Your analysis may be correct, but implementation leads something to be
>desired - what happens if we exit the function just above this deleted
>line - returning -EIO ?  Shouldn't there be some releasing of the
>clock hold?

Hi Russell,

Thanks a lot for your kind help and also your review.
I'll fix it in next version.

Best regards,
Vincent Yang

>
>--
>FTTC broadband for 0.8mile line: currently at 9.5Mbps down 400kbps up
>according to speedtest.net.