Re: [PATCHv6 2/5] clk: socfpga: Add a clock type for the SD/MMC driver

["David Lanzendörfer" <david.lanzendoerfer@o2s.ch>]

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Hi
> that takes the MMC clock (and only the MMC clock) and does the setup
> (it's basically configuring two values, "sample" and "output", into the
> clock register). I really don't know what does this do/why is it
> required/when is it used; I'm cc'ing Hans and David who can hopefully
> explain that part.
Since the read/write operations have to happen asynchronously and in a manner
so that the data provided by the SD-Card can be fetched on time, a specific
clock phase shift is required as you can see in the linked picture[1].
That's what the function is doing: It accesses the special registers of the
MMC clock device and configures this phase offset.

> I also saw a similar requirement from the gmac people (on cc too), who
> needed to set the phy type (or something like that) on one of the clock
> registers; so I'm thinking maybe a generic "tune something" approach
> that lets users configure some clock-dependent, arbitrary aspect of the
> clock is the way forward. Otherwise, we're going to end up bloating the
> clock framework with a lot of callback pointers that are going to be
> used in one or two clocks out of potentially hundreds on the whole system.
It totally makes sense, since GMAC will be doing asynchrous
operations as well.

-David

[1] http://www.chipestimate.com/techtalk/images/11022010_fig2_3.JPG

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