Re: [PATCH v2 1/2] clocksource/drivers/exynos_mct_v2: introduce Exynos MCT version 2 driver for next Exynos SoC

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Nov 03, 2021 at 06:57:28PM +0900, Youngmin Nam wrote:
> On Wed, Nov 03, 2021 at 10:04:36AM +0100, Krzysztof Kozlowski wrote:
> > On 03/11/2021 10:24, Youngmin Nam wrote:
> > > On Wed, Nov 03, 2021 at 09:18:07AM +0100, Krzysztof Kozlowski wrote:
> > >> On 03/11/2021 01:09, Youngmin Nam wrote:
> > >>> On Tue, Nov 02, 2021 at 10:28:10AM +0000, Mark Rutland wrote:
> > >>>> On Tue, Nov 02, 2021 at 09:11:21AM +0900, Youngmin Nam wrote:

> > >>>>> +	evt->rating = 500;	/* use value higher than ARM arch timer */
> > >>>>
> > >>>> Previously Will asked you to try CLOCK_EVT_FEAT_PERCPU here, and to set
> > >>>> the C3STOP flag on the arch timer via the DT when necessary, rather than
> > >>>> trying to override the arch timer like this:
> > >>>>
> > >>>>   https://protect2.fireeye.com/v1/url?k=72526080-2dc9598b-7253ebcf-002590f5b904-ca603717c6462908&q=1&e=be56aa83-dbac-4639-913d-d388620fe3fc&u=https%3A%2F%2Flore.kernel.org%2Fr%2F20211027073458.GA22231%40willie-the-truck

> > >>> Hi Mark.
> > >>> It looks like you missed my previous mail.
> > >>> https://protect2.fireeye.com/v1/url?k=ab15817a-cbf71c27-ab140a35-000babd9f1ba-123b7f313b1b1ccc&q=1&e=34c8716e-6d2e-4d8e-82fe-04777ebc5eb3&u=https%3A%2F%2Flore.kernel.org%2Fall%2F20211029035422.GA30523%40perf%2F%23t
> > >>>
> > >>> Yes, I believe Will's suggestion definitely will work.

Then please do so.

> > >>> But that is for performance not functionality.

No; it's about *consistency*, and avoiding unnecssary special cases. The
whole point is that marking the generic timer as C3STOP *accurately*
describes how the timer behaves on your platform, and marking the MCTv2
as a percpu timer which *can* act as a back-up also aligns with that.

That approach leaves the policy in the kernel, and we can play about
with that later without functional breakage.

> > >>> As a driver for new H/W IP I would like to confirm functionality first.
> > >>> We need more time to test this feature with our exynos core power down feature.
> > >>> And we need to do a various regression test whether there is another corner case or not.
> > >>> So, how about we apply Will's suggetion later after the current patchset is merged first?
> > >>> After doing our regression test with our exynos core power down feature, we can confirm this.
> > >>
> > >> Not really, because once it is merged there is no incentive to fix it or
> > >> simply changing it can be forgotten. Also similarly to commit
> > >> 6282edb72bed ("clocksource/drivers/exynos_mct: Increase priority over
> > >> ARM arch timer"), there should be a valid and serious reason to
> > >> prioritize Exynos MCT.

It's also worth nothing that the case described for 6282edb72bed is
really a system design erratum, since the counter is supposed to be in
an always-on power domain and should be counting well before a regular
OS kernel boots. The arm64 kernel requires the architected counter to be
running before it is entered, or there will be subtle breakage.

> > > No, it's not. I also want to decrease MCTv2 timer rating so that we want to use arm arch timer as a default.
> > > But this feature has to be confirmed with core power down feature enabled.
> > > Without core power down feature, we can't comfirm this.
> > > Ater that we need to check whether there is regression or not related power, stability, and so on.
> > > I'm not saying I will not apply Will's suggestion but I just want to apply later after some hard test.
> > 
> > You repeat the same argument, the same words. Nothing new. Repeating the
> > same won't change it, use the lower priority. This is a patch for new
> > kernel, so there is a plenty of time to test it and it won't affect your
> > production environment.
> > 
> So, how about we control timer rating value with DT ?
> Of course the default rating value should be lower than arm arch timer's.
> Do you agree with this?

No; placing a rating value in the DT is a hack. That should *not* live
in the DT because it's linux-internal detail and not a description of
the HW.

Thanks,
Mark.