[U-Boot] [PATCH v4 2/3] mmc: dw_mmc: Support bypass mode with the get_mmc_clk() method

[Simon Glass]

Hi Marek,

On 12 August 2015 at 07:53, Marek Vasut <marex at denx.de> wrote:
> On Wednesday, August 12, 2015 at 03:51:07 PM, Simon Glass wrote:
>> Hi Marek,
>>
>> On 12 August 2015 at 07:48, Marek Vasut <marex at denx.de> wrote:
>> > On Wednesday, August 12, 2015 at 03:04:15 PM, Simon Glass wrote:
>> >> Hi Marek,
>> >
>> > Hi!
>> >
>> > [...]
>> >
>> >> >> >> > Why are you passing the @freq into get_mmc_clk() ? Shouldn't you
>> >> >> >> > call some clock framework function to determine the input
>> >> >> >> > frequency of the DWMMC block from within the get_mmc_clk()
>> >> >> >> > implementation instead ? What do you think please ?
>> >> >> >>
>> >> >> >> Well, yes. If such a clock frame work existed I would call it :-)
>> >> >> >> We do have a uclass now so we are getting there.
>> >> >> >
>> >> >> > Excellent, so do you really need this kind of patch ? :) Why don't
>> >> >> > you make just some kind of function -- get_dwmmc_clock() -- and
>> >> >> > call it instead ?
>> >> >>
>> >> >> This is sort-of what is happening. It is calling a function in the
>> >> >> host controller - i.e. the SoC's MMC controller. It is one step
>> >> >> closer to knowing the input clock to the dwmmc input clock. Note
>> >> >> that it is not the clock of the MMC bus itself, but the input clock
>> >> >> to the dwmmc logic block.
>> >> >
>> >> > I don't think I quite understand what you mean here. We're talking
>> >> > about obtaining the frequency of the clock which go into the DWMMC IP
>> >> > block, right ?
>> >> >
>> >> > So, if you implement a function, say -- dwmmc_get_upstream_clock() --
>> >> > and call it from within the implementation of the .get_mmc_clk(),
>> >> > which is specific for that particular chip of yours*, you don't need
>> >> > this patch. Or am I really missing something fundamental ?
>> >> >
>> >> > *the .get_mmc_clk() is specific to a chip, see for example
>> >> > exynos_dw_mmc.c
>> >>
>> >> The purpose of the existing code (before my change) is to find out the
>> >> input frequency of the IP block. By knowing this, the dw_mmc driver
>> >> can work out what divisor it needs to achieve a particular MMC bus
>> >> clock.
>> >>
>> >> The implementation of get_mmc_clk() (which will be in the SoC-specific
>> >> MMC driver) is indeed the place where the clock is figured out. My
>> >> only change is to add a parameter which is the desired bus clock. This
>> >> parameter can be ignored, but for implementations which can select the
>> >> source clock such that it matches this bus clock, then they can do
>> >> this and dw_mmc can just use bypass mode.
>> >
>> > I see now, this wasn't really clear from the patch description. Shouldn't
>> > you introduce another callback for this purpose then, like .set_mmc_clk()
>> > instead ?
>>
>> We could do, but I don't like introducing another interface for one
>> client. Also I think the right solution is to move it to use the
>> generic clock infrastructure, when it exists (well we have it, but
>> nothing uses it yet).
>
> OK, but making a .get_mmc_clk() function actually configure something
> is a behavior I wouldn't expect from a getter function. It's a bit odd
> and illogical in my opinion.

Yes fair enough, it is odd. I did start an MMC uclass so perhaps that
will lead to a better solution. It's unfortunately that dw_mmc need
its own callback infrastructure.

Regards,
Simon