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

[Stephen Boyd]

Quoting Shawn Lin (2018-04-05 07:47:41)
> Hi Heiko,
> 
> On 2018/4/5 21:30, Heiko Stuebner wrote:
> > 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.
> 
> Ahh, really good catch, and I always forgot 1 is the power of 2.
> 
> More or less, it's a hardware property for Rockchip as these divders
> only hardwired to specific controllers which always request even div
> field, even if the div field could be odd number physcially. But yes,
> the common case wouldn't be that, and another combination would be
> CLK_DIVIDER_EVEN | CLK_DIVIDER_ONE_BASED?
> 

Maybe you can use a table of divider values? That is a simple escape
hatch for this sort of thing and doesn't require adding another flag to
the divider code to handle this.

Otherwise it would be good to handle these combinations of flags as
well. We're only up to 7 flags so it isn't too bad yet.