Re: [PATCH v2 1/2] clk: change clk_ops' ->round_rate() prototype

[Jon Hunter <jonathanh@nvidia.com>]

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@nvidia.com> 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