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

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Message-ID: <20240321-faithful-caterpillar-of-health-52d87f@houat>
Date: Thu, 21 Mar 2024 17:24:58 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Jernej Škrabec <jernej.skrabec@...il.com>
Cc: Frank Oltmanns <frank@...manns.dev>, Chen-Yu Tsai <wens@...e.org>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Samuel Holland <samuel@...lland.org>, Icenowy Zheng <uwu@...nowy.me>, Ondrej Jirman <x@...x.eu>, 
	dri-devel@...ts.freedesktop.org, linux-arm-kernel@...ts.infradead.org, 
	linux-sunxi@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/sun4i: tcon: Support keeping dclk rate upon ancestor
 clock changes

On Thu, Mar 14, 2024 at 06:20:58PM +0100, Jernej Škrabec wrote:
> Dne četrtek, 14. marec 2024 ob 15:42:24 CET je Maxime Ripard napisal(a):
> > 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@...manns.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.

IIRC, it was because the D-PHY and DSI controller derive from the same
clock, and we needed to make sure setting one wouldn't affect the other
one.

Maxime

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