Re: [PATCH 1/2] clk: divider: Add CLK_DIVIDER_EVEN flag support

[Heiko Stuebner <heiko@sntech.de>]

Hi Shawn,

Am Donnerstag, 5. April 2018, 07:38:18 CEST schrieb Shawn Lin:
> CLK_DIVIDER_EVEN is used for clock divders that should only
> use even number in the div field.
> 
> Two clock divder should consider to use this flag
> (1) The divder is physically only support even div number to
> generate 50% duty cycle clock rate.
> (2) The divder's clock consumer request it to use even div number
> to generate the most closest requested rate for whatever reason.
> 
> In some platforms, for instance Rockchip, the eMMC/SDIO/SDMMC should
> request divder to use even number when working at a high throughput
> speed mode reliably. However, that wasn't guaranteed by clock framework.
> So the previous tricky is to carefully assign magic clock rate to their
> parents as well as consumer's clock rate when requesting. That works
> bad in practice if folks change the parents clock rate or the clock
> hierarchy randomly. That also work bad if the consumer's clock rate
> came from the DT, which is changed so fraquent for different boards.
> 
> To make it's less prone to make mistake and to make it really respect
> the fact that the divder should use even number to the div field, we
> need the clock framework's help. Now we have CLK_DIVIDER_POWER_OF_TWO,
> which could guarantee the div field is even number, however, obviously
> it skips the even numbers which is the power of 2, but maybe which is
> the best div to generate closest clock rate for consumer.

I think there is a slight misunderstanding here, CLK_DIVIDER_POWER_OF_TWO
means "2 raised to the value read from the hardware register", so describes
a hardware property, while your CLK_DIVIDER_EVEN describes needed special
handling due to some hardware necessity.

That is not meant to nack your change, but just to point out that you can
also have CLK_DIVIDER_POWER_OF_TWO | CLK_DIVIDER_EVEN, which is
relevant for the uneven 2^0 = 1 case and should be taken in to account.


Heiko