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

[Marek Vasut <marex@denx.de>]

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 ?

> Unless we pass the bus frequency to get_mmc_clk() it has no way of
> knowing what bus clock is required and thus cannot implement this
> feature. The feature implementation is entirely within the
> implementation of get_mmc_clk() - it just needs one more piece of
> information to do its job.

I see, thanks for clearing this up!