On 08/09/2013 04:41 PM, Dinh Nguyen wrote: > 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. I'm not sure "bus-hz" is the right name; it's not clear from that name that it refers to an internal clock inside the MMC controller rather than the SD bus out to the card itself. Perhaps "controller-clock-hz" or something like that? -- 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
