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

[Stephen Warren <swarren@wwwdotorg.org>]

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.