linux-kernel - Re: [PATCH v4 21/27] clk: bcm: rpi: Discover the firmware clocks

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Date:   Thu, 11 Jun 2020 13:14:58 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     linux-rpi-kernel@...ts.infradead.org,
        bcm-kernel-feedback-list@...adcom.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Tim Gover <tim.gover@...pberrypi.com>,
        Phil Elwell <phil@...pberrypi.com>,
        Mike Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>, linux-clk@...r.kernel.org
Subject: Re: [PATCH v4 21/27] clk: bcm: rpi: Discover the firmware clocks

On Thu, 2020-06-11 at 09:32 +0200, Maxime Ripard wrote:
> The RaspberryPi4 firmware actually exposes more clocks than are currently
> handled by the driver and we will need to change some of them directly
> based on the pixel rate for the display related clocks, or the load for the
> GPU.
> 
> Since the firmware implements DVFS, this rate change can have a number of
> side-effects, including adjusting the various PLL voltages or the PLL
> parents. The firmware also implements thermal throttling, so even some
> thermal pressure can change those parameters behind Linux back.
> 
> DVFS is currently implemented on the arm, core, h264, v3d, isp and hevc
> clocks, so updating any of them using the MMIO driver (and thus behind the
> firmware's back) can lead to troubles, the arm clock obviously being the
> most problematic.
> 
> In order to make Linux play as nice as possible with those constraints, it
> makes sense to rely on the firmware clocks as much as possible. However,
> the firmware doesn't seem to provide some equivalents to their MMIO
> counterparts, so we can't really replace that driver entirely.
> 
> Fortunately, the firmware has an interface to discover the clocks it
> exposes.
> 
> Let's use it to discover, register the clocks in the clocks framework and
> then expose them through the device tree for consumers to use them.
> 
> Cc: Michael Turquette <mturquette@...libre.com>
> Cc: Stephen Boyd <sboyd@...nel.org>
> Cc: linux-clk@...r.kernel.org
> Reviewed-by: Stephen Boyd <sboyd@...nel.org>
> Signed-off-by: Maxime Ripard <maxime@...no.tech>
> ---

Acked-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>

Regards,
Nicolas


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