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

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Message-ID: <20210503083221.qsdurp2f3bkwfa6d@gilmour>
Date:   Mon, 3 May 2021 10:32:21 +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,

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?

Maxime

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