linux-kernel - Re: Export clocks_calc_mult

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Message-ID: <CALAqxLUWzUYWBxJwMzi5cw2LgcQC6tFc_j4pHGmKh+QwhDMKZA@mail.gmail.com>
Date:	Wed, 30 Mar 2016 10:01:09 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Murali Karicheri <m-karicheri2@...com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Kristo, Tero" <t-kristo@...com>
Subject: Re: Export clocks_calc_mult_shift() function

On Wed, Mar 30, 2016 at 9:46 AM, Murali Karicheri <m-karicheri2@...com> wrote:
> On 03/14/2016 11:27 AM, Murali Karicheri wrote:
>> On 03/08/2016 05:25 PM, Murali Karicheri wrote:
>>> Hi,
>>>
>>> I found a patch posted sometime back to export the clocksource
>>> function clocks_calc_mult_shift() so that it can be called by
>>> drivers that are dynamically loadable. I have not seen any
>>> comment against this. Wondering why this is not merged. We require
>>> this function exported for use in our driver as well. Can you merge
>>> the patch please. Or do you suggest me to repost the same?
>>>
>>> http://lkml.iu.edu/hypermail/linux/kernel/1502.2/01641.html
>>>
>>> Thanks
>>>
>> John,
>>
>> Some reason, your response didn't make into my inbox. So I am
>> reproducing it below.
>>
>>> Why would the clocksource driver need to calculate the hz/shift value
>>> instead of using the clocksource_register_hz/khz functions?
>>>
>>> thanks
>>> -john
>>
>> John,
>>
>> In this use case, the timestamp for Tx/Rx is generated by a firmware
>> that attach the timestamp raw count to the packet meta data when the
>> same is received from the Packet Accelerator h/w at the ingress.
>> We need to convert this raw count value to nano second and use a code
>> like this.
>>
>> /* Convert a raw PA timer count to nanoseconds
>>  */
>> static inline u64 tstamp_raw_to_ns(struct pa_core_device *core_dev, u32 lo,
>>                                    u32 hi)
>> {
>>         u32 mult = core_dev->timestamp_info.mult;
>>         u32 shift = core_dev->timestamp_info.shift;
>>         u64 result;
>>
>>         /* Minimize overflow errors by doing this in pieces */
>>         result  = ((u64)lo * mult) >> shift;
>>         result += ((u64)hi << (32 - shift)) * mult;
>>
>>         return result;
>> }
>>
>> The mult, shift values are obtained using the existing clocks_calc_mult_shift()
>> that will not work, if our driver is built as a dynamically loadable module
>> as the symbol is not exported.
>>
>> Is there an alternative way of doing this without exporting this function.
>> clocksource_register_hz/khz() can't help in this, right?
>>
> John,
>
> I didn't see any response? Can I send a patch to export clocks_calc_mult_shift() function??

Sorry. I was out at a conference when you first sent this out and then
I just now got back from vacation.

I didn't realize this was for the

So I'm still hesitant to export that function because its a bit too
raw and I'm not sure drivers should be trusted to handle the inherent
tradeoffs it makes (adjustability and accuracy vs counter delta limits
to avoid mult overflow) properly without much guidance.

Could you instead export a helper function like what
clocksource_register_hz/khz() provides? So the timecounter drivers get
usable values w/o having to sort out the interval length (as it
probably would just be blindly copy-pasted in from somewhere else).

thanks
-john