Re: [PATCH] mmc: dw_mmc: add fixed divider for ciu_clk on SoCFPGA

From: Doug Anderson
Date: Fri Apr 10 2015 - 14:21:15 EST


On Fri, Apr 10, 2015 at 11:08 AM, Dinh Nguyen <dinh.linux@xxxxxxxxx> wrote:
> On 4/10/15 10:15 AM, Doug Anderson wrote:
>> Dinh,
>> On Fri, Apr 10, 2015 at 6:56 AM, <dinguyen@xxxxxxxxxxxxxxxxxxxxx> wrote:
>>> From: Dinh Nguyen <dinguyen@xxxxxxxxxxxxxxxxxxxxx>
>>> The ciu_clk(Card Interface Unit Clock) on the SoCFPGA platform has a fixed
>>> divider of 4. Add the fixed clock divide code in the platform's clock
>>> setup code.
>> It might actually be better to do this a different way for SoCFPGA. I
>> sorta wish we had done it differently for Rockchip as well, but at
>> this point you end up with the complexity of changing device tree
>> bindings in conjunction with code and it gets ugly.
> Yes, I started going down this path and realized that.
>> Specifically, you've probably got the following clocks:
>> SD_prediv = 400MHz
>> -> SD postdiv = 100MHz
>> -> SD sample = 100MHz, shifted
>> -> SD drive = 100MHz, shifted
>> Right now you're specifying "SD_prediv" as the SD card clock. If you
>> instead expose "SD postdiv" as a new clock (from your clock driver)
>> that is "SD prediv" divided by 4 then you'll magically get all the
>> behavior that you want with no modifications to dw_mmc. Just make
>> sure that "SD postdiv" passes on rate changes to its parent (that's
>> just a flag in the common clock framework).
> That's a great idea, thanks for pointing that out.
>> At some point in time you'll also want to expose the sample and drive
>> clocks once you get UHS modes working. Alexandru posted some patches
>> for this a while ago to support tuning in dw_mmc using just drive and
>> sample clocks, but the patch still needed some more work. Either he
>> or I will probably pick it up again soon.
> I think I have already done this by representing the sdmmc_clk with a
> "clk-phase" property for this clock.

I think there needs to be a separate sample and drive clock at least,
since they can have different phases, right? I haven't looked, but if
that's what you have then you're all set! :) Above I was suggesting
a 3rd clock instead of arbitrarily using either the sample/drive clock
as the main clock for dw_mmc.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at