Hi Boris, On 05/06/15 12:39, Boris Brezillon wrote: > Hi Jon, > > On Fri, 5 Jun 2015 09:46:09 +0100 > Jon Hunter <jonathanh@xxxxxxxxxx> wrote: > >> >> On 05/06/15 00:02, Paul Walmsley wrote: >>> Hi folks >>> >>> just a brief comment on this one: >>> >>> On Thu, 30 Apr 2015, Boris Brezillon wrote: >>> >>>> Clock rates are stored in an unsigned long field, but ->round_rate() >>>> (which returns a rounded rate from a requested one) returns a long >>>> value (errors are reported using negative error codes), which can lead >>>> to long overflow if the clock rate exceed 2Ghz. >>>> >>>> Change ->round_rate() prototype to return 0 or an error code, and pass the >>>> requested rate as a pointer so that it can be adjusted depending on >>>> hardware capabilities. >>> >>> ... >>> >>>> diff --git a/Documentation/clk.txt b/Documentation/clk.txt >>>> index 0e4f90a..fca8b7a 100644 >>>> --- a/Documentation/clk.txt >>>> +++ b/Documentation/clk.txt >>>> @@ -68,8 +68,8 @@ the operations defined in clk.h: >>>> int (*is_enabled)(struct clk_hw *hw); >>>> unsigned long (*recalc_rate)(struct clk_hw *hw, >>>> unsigned long parent_rate); >>>> - long (*round_rate)(struct clk_hw *hw, >>>> - unsigned long rate, >>>> + int (*round_rate)(struct clk_hw *hw, >>>> + unsigned long *rate, >>>> unsigned long *parent_rate); >>>> long (*determine_rate)(struct clk_hw *hw, >>>> unsigned long rate, >>> >>> I'd suggest that we should probably go straight to 64-bit rates. There >>> are already plenty of clock sources that can generate rates higher than >>> 4GiHz. >> >> An alternative would be to introduce to a frequency "base" the default >> could be Hz (for backwards compatibility), but for CPUs we probably only >> care about MHz (or may be kHz) and so 32-bits would still suffice. Even >> if CPUs cared about Hz they could still use Hz, but in that case they >> probably don't care about GHz. Obviously, we don't want to break DT >> compatibility but may be the frequency base could be defined in DT and >> if it is missing then Hz is assumed. Just a thought ... > > Yes, but is it really worth the additional complexity. You'll have to > add the unit information anyway, so using an unsigned long for the > value and another field for the unit (an enum ?) is just like using a > 64 bit integer. For a storage perspective, yes it would be the same. However, there are probably a lot of devices that would not need the extra range, but would now have to deal with 64-bit types. I have no idea how much overhead that would be in reality. If the overhead is negligible then a 64-bit type is definitely the way to go, as I agree it is simpler and cleaner. Cheers Jon
