Because some SDIO devices clock are below 50MHz , Not as IMX51/53, we can set the Max SD clock in board file.
In I.MX6 , there is no Max. Clock setting for SDIO.
Yes, correct for i.MX6 there is no max. clock settings because
every ARM sub-arch has a different, incompatible struct clk implementation.
Seems you can add such max. restriction in function _clk_usdhc_round_rate
More about Linux clock architecture one can find on eLinux.org:
"Common clock framework: how to use it" - eLinux.org
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