On Fri, 2013-08-09 at 15:00 -0600, Stephen Warren wrote: > On 08/08/2013 05:10 PM, Dinh Nguyen wrote: > > On Thu, 2013-08-08 at 15:13 -0600, Stephen Warren wrote: > ... > >> Why is there a need to directly represent the divider anywhere? The > >> driver can find the rate of the input clock, and I assume it knows what > >> rate it wants the clock to run at, so can't it calculate the divider > >> based on those two pieces of information? > > > > CC: Chris Ball > >> > >> Or, does the driver really not know what rate it wants the clock to be > >> after the internal divider? If not, then I think that *rate* should be > >> recorded in DT, not the divider to obtain that rate. > >> > > > > I believe that this is the case, that the driver does not know what rate > > it will clock the SD card at. I think internally we did have a "bus_hz" > > in DT a while back. I guess I don't really understand why it would be > > better to have a *rate* DT entry instead of a fixed-divider entry? > > The value of the divider depends on two things: > > 1) Input clock rate. > 2) Desired rate after applying the internal divider. > > The input clock rate may vary, either between SoCs the IP is integrated > into, or even at run-time perhaps base on clk_set_rate() etc. > > If the MMC driver knows the clock rate it wants to run at, it can > calculate the divider easily; it can automatically adjust to any input > clock that its environment may provide. > > If the MMC driver is simply told "use this divider", that's encoding > assumptions about the rate of the input clock which might not be valid;. > Encoding the desire clock rate within the MMC HW block allows the > divider to be calculated based on the actual environment. Thanks Stephen. That makes sense. Chris, Jaehoon, and Seungwon, do you have any inputs? If not I will go down the path of have the "bus-hz" in the DTS node for the clock rate of the CIU clock. Then I would also make the same change to dw_mmc-exynos but would need your help on what the rate would be. For SOCFPGA, its 12.5 MHZ. Thanks, Dinh > -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
