Re: [PATCHv2] ARM: socfpga: dts: Add support for SD/MMC

[Stephen Warren <swarren@wwwdotorg.org>]

On 08/09/2013 04:41 PM, Dinh Nguyen wrote:
> On Fri, 2013-08-09 at 15:00 -0600, Stephen Warren wrote:
>> On 08/08/2013 05:10 PM, Dinh Nguyen wrote:
>>> On Thu, 2013-08-08 at 15:13 -0600, Stephen Warren wrote:
>> ...
>>>> Why is there a need to directly represent the divider anywhere? The
>>>> driver can find the rate of the input clock, and I assume it knows what
>>>> rate it wants the clock to run at, so can't it calculate the divider
>>>> based on those two pieces of information?
>>>
>>> CC: Chris Ball
>>>>
>>>> Or, does the driver really not know what rate it wants the clock to be
>>>> after the internal divider? If not, then I think that *rate* should be
>>>> recorded in DT, not the divider to obtain that rate.
>>>>
>>>
>>> I believe that this is the case, that the driver does not know what rate
>>> it will clock the SD card at. I think internally we did have a "bus_hz"
>>> in DT a while back. I guess I don't really understand why it would be
>>> better to have a *rate* DT entry instead of a fixed-divider entry?
>>
>> The value of the divider depends on two things:
>>
>> 1) Input clock rate.
>> 2) Desired rate after applying the internal divider.
>>
>> The input clock rate may vary, either between SoCs the IP is integrated
>> into, or even at run-time perhaps base on clk_set_rate() etc.
>>
>> If the MMC driver knows the clock rate it wants to run at, it can
>> calculate the divider easily; it can automatically adjust to any input
>> clock that its environment may provide.
>>
>> If the MMC driver is simply told "use this divider", that's encoding
>> assumptions about the rate of the input clock which might not be valid;.
>> Encoding the desire clock rate within the MMC HW block allows the
>> divider to be calculated based on the actual environment.
> 
> Thanks Stephen. That makes sense.
> 
> Chris, Jaehoon, and Seungwon, do you have any inputs? If not I will go
> down the path of have the "bus-hz" in the DTS node for the clock rate of
> the CIU clock. Then I would also make the same change to dw_mmc-exynos
> but would need your help on what the rate would be.
> 
> For SOCFPGA, its 12.5 MHZ.

I'm not sure "bus-hz" is the right name; it's not clear from that name
that it refers to an internal clock inside the MMC controller rather
than the SD bus out to the card itself. Perhaps "controller-clock-hz" or
something like that?