[U-Boot] [PATCH] mmc: Set the initial clock speed to 400KHz

[Jaehoon Chung]

Hi Phil,

On 05/26/2017 05:01 PM, Phil Edworthy wrote:
> Hi Jaehoon Chung,
> 
> On 26 May 2017 04:38 Jaehoon Chung wrote:
>> On 05/25/2017 11:14 PM, Phil Edworthy wrote:
>>> On 25 May 2017 15:10 Jaehoon Chung wrote:
>>>> On 05/25/2017 11:02 PM, Phil Edworthy wrote:
>>>>> On 25 May 2017 14:50 Jaehoon Chung wrote:
>>>>>> On 05/24/2017 10:54 PM, Phil Edworthy wrote:
>>>>>>> The code currently defaults to the slowest clock speed that can be
>>>>>>> achieved, which can be significantly lower than the SD spec.
>>>>>>
>>>>>> Is there any problem..As i know, it should be changed from 1 to
>> min_clk.
>>>>> The only problem is that the initial SD clock can be very slow so it
>>>>> increases the time to start SD. Admittedly, it's a very small
>>>>> increase in time, but we should use the correct initial clock speed.
>>>>
>>>> Well..i didn't agree yet...
>>>>
>>>> If mmc_set_clock(mmc, 400K) and mmc->cfg->f_min is 300K?
>>>> Initial clock should be always 400K..but spec is mentioned "initial
>>>> clock is maximum 400K.."
>>>> It means the clock can be the value under 400K.
>>> I'm not sure I follow you.
>>> The spec means that all SD cards must support an initial clock speed of
>> 400KHz, right?
>>
>> No..clock frequency range shall be 100KHz - 400KHz during initialization
>> sequence.
>> Some targets should not work fine with 400KHz..So it needs to check f_min
>> value.
>> otherwise, it needs to find the initial clock value like Linux kernel scheme.
> f_min is not the frequency that must be used during init, it is the minimum
> frequency that the Controller can operate at. If a controller has f_min=300KHz
> then there is no problem attempting to set the MMC clock to 400KHz. If the
> controller cannot do exactly 400KHz, the driver should set the clock to the
> closest frequency _under_ 400KHz.
> 
> For example, SDHCI v3 controllers set f_min to f_max / 2046. In my particular
> case, SDHCI v3 controller f_max is 50MHz, so f_min is approximately 25KHz.
> Therefore, when the code does this:
> 	mmc_set_clock(mmc, 1);
> it will set the clock rate to approximately 25KHz.
> Clearly this is outside the valid 100-400KHz range that you quote.

Well..then i will check the real frequency with oscilloscope..
then it's more clear than now.. :)

Thanks for pointing out about this.

Best Regards,
Jaehoon Chung

> 
> Thanks
> Phil
> 
>>> If so, then there is no harm in setting it to 400KHz.
>>>
>>> Thanks
>>> Phil
>>>
>>>>>>> Signed-off-by: Phil Edworthy <phil.edworthy at renesas.com>
>>>>>>> ---
>>>>>>>  drivers/mmc/mmc.c | 2 +-
>>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c index
>>>>>>> 72fc177..dff1be3 100644
>>>>>>> --- a/drivers/mmc/mmc.c
>>>>>>> +++ b/drivers/mmc/mmc.c
>>>>>>> @@ -1676,7 +1676,7 @@ int mmc_start_init(struct mmc *mmc)
>> #endif
>>>>>>>  	mmc->ddr_mode = 0;
>>>>>>>  	mmc_set_bus_width(mmc, 1);
>>>>>>> -	mmc_set_clock(mmc, 1);
>>>>>>> +	mmc_set_clock(mmc, 400000);
>>>>>>>
>>>>>>>  	/* Reset the Card */
>>>>>>>  	err = mmc_go_idle(mmc);
>>>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>
>>>
>>>
>>>
> 
> 
> 
>