linux-kernel - Re: [PATCH 0/2] clk: Implement a clock request API

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Date:   Mon, 24 May 2021 14:48:11 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Mike Turquette <mturquette@...libre.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org, Eric Anholt <eric@...olt.net>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        Russell King <linux@...linux.org.uk>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Phil Elwell <phil@...pberrypi.com>,
        Tim Gover <tim.gover@...pberrypi.com>,
        Dom Cobley <dom@...pberrypi.com>, linux-clk@...r.kernel.org
Subject: Re: [PATCH 0/2] clk: Implement a clock request API

Hi Stephen, Mike,

On Mon, May 03, 2021 at 10:32:21AM +0200, Maxime Ripard wrote:
> Hi Stephen,
> 
> On Fri, Apr 30, 2021 at 01:59:39PM -0700, Stephen Boyd wrote:
> > Quoting Maxime Ripard (2021-04-13 03:13:18)
> > > Hi,
> > > 
> > > This is a follow-up of the discussion here:
> > > https://lore.kernel.org/linux-clk/20210319150355.xzw7ikwdaga2dwhv@gilmour/
> > > 
> > > This implements a mechanism to raise and lower clock rates based on consumer
> > > workloads, with an example of such an implementation for the RaspberryPi4 HDMI
> > > controller.
> > > 
> > > There's a couple of things worth discussing:
> > > 
> > >   - The name is in conflict with clk_request_rate, and even though it feels
> > >     like the right name to me, we should probably avoid any confusion
> > > 
> > >   - The code so far implements a policy of always going for the lowest rate
> > >     possible. While we don't have an use-case for something else, this should
> > >     maybe be made more flexible?
> > 
> > I'm definitely confused how it is different from the
> > clk_set_rate_exclusive() API and associated
> > clk_rate_exclusive_get()/clk_rate_exclusive_put(). Can you explain
> > further the differences in the cover letter here?
> 
> The exclusive API is meant to prevent the clock rate from changing,
> allowing a single user to make sure that no other user will be able to
> change it.
> 
> What we want here is instead to allow multiple users to be able to
> express a set of minimum rates and then let the CCF figure out a rate
> for that clock that matches those constraints (so basically what
> clk_set_min_rate does), but then does allow for the clock to go back to
> its initial rate once that constraint is not needed anymore.
> 
> So I guess it's more akin to clk_set_min_rate with rollback than the
> exclusive API?

Is that rationale good enough, or did you expect something else?

Maxime

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