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    Could you please tell me the requirement of "op_code" parameter in
    mmc_send_tuning()?
    According to mmc_send_tuning(),it seems that tuning command(CMD19/CMD21)
    is required. Thus device will not response mmc_send_tuning() if current
    speed mode doesn't support tuning command.
    Please correct me if I am wrong.

When the mmc core decides it's time to execute tuning, it invokes the
->execute_tuning() host ops, which has the "opcode" as a parameter.
You should be able to use it when calling mmc_send_tuning().

[...]

Kind regards
Uffe