Re: [PATCH] drm/sun4i: tcon: Support keeping dclk rate upon ancestor clock changes

From: "Jernej Škrabec" <jernej.skrabec@gmail.com>
To: Frank Oltmanns <frank@oltmanns.dev>, Maxime Ripard <mripard@kernel.org>
Cc: Chen-Yu Tsai <wens@csie.org>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
	Samuel Holland <samuel@sholland.org>,
	Icenowy Zheng <uwu@icenowy.me>, Ondrej Jirman <x@xnux.eu>,
	dri-devel@lists.freedesktop.org,
	linux-arm-kernel@lists.infradead.org,
	linux-sunxi@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] drm/sun4i: tcon: Support keeping dclk rate upon ancestor clock changes
Date: Thu, 14 Mar 2024 18:20:58 +0100	[thread overview]
Message-ID: <5448341.Sb9uPGUboI@jernej-laptop> (raw)
In-Reply-To: <20240314-careful-silky-bear-8ee43f@houat>

Dne četrtek, 14. marec 2024 ob 15:42:24 CET je Maxime Ripard napisal(a):
> Hi,
> 
> On Sun, Mar 10, 2024 at 02:32:29PM +0100, Frank Oltmanns wrote:
> > Allow the dclk to reset its rate when a rate change is initiated from an
> > ancestor clock. This makes it possible to no longer to get an exclusive
> > lock. As a consequence, it is now possible to set new rates if
> > necessary, e.g. when an external display is connected.
> > 
> > The first user of this functionality is the A64 because PLL-VIDEO0 is an
> > ancestor for both HDMI and TCON0. This allows to select an optimal rate
> > for TCON0 as long as there is no external HDMI connection. Once a change
> > in PLL-VIDEO0 is performed when an HDMI connection is established, TCON0
> > can react gracefully and select an optimal rate based on this the new
> > constraint.
> > 
> > Signed-off-by: Frank Oltmanns <frank@oltmanns.dev>
> > ---
> > I would like to make the Allwinner A64's data-clock keep its rate
> > when its ancestor's (pll-video0) rate changes. Keeping data-clock's rate
> > is required, to let the A64 drive both an LCD and HDMI display at the
> > same time, because both have pll-video0 as an ancestor.
> > 
> > TCONs that use this flag store the ideal rate for their data-clock and
> > subscribe to be notified when data-clock changes. When rate setting has
> > finished (indicated by a POST_RATE_CHANGE event) the call back function
> > schedules delayed work to set the data-clock's rate to the initial value
> > after 100 ms. Using delayed work maks sure that the clock setting is
> > finished.
> > 
> > I've implemented this functionality as a quirk, so that it is possible
> > to use it only for the A64.
> > 
> > This patch supersedes [1].
> > 
> > This work is inspired by an out-of-tree patchset [2] [3] [4].
> > Unfortunately, the patchset uses clk_set_rate() directly in a notifier
> > callback, which the following comment on clk_notifier_register()
> > forbids: "The callbacks associated with the notifier must not re-enter
> > into the clk framework by calling any top-level clk APIs." [5]
> > Furthermore, that out-of-tree patchset no longer works since 6.6,
> > because setting pll-mipi is now also resetting pll-video0 and therefore
> > causes a race condition.
> 
> Workqueues don't have an upper boundary on when they execute. As we
> discussed multiple times, this should be solved in the clock framework
> itself, not bypassing it.

I think TCON code still needs to be touched due to clk_rate_exclusive_get()
calls which effectively lock whole chain. You can't have both TCONs locking
rate on A64 for this to work correctly.

What was original reason for clk_rate_exclusive_get()? I forgot already.

Best regards,
Jernej

> 
> Maxime
> 

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