Re: [PATCH] drm/panel: move some dsi commands from unprepare to disable

From: Linus Walleij <linus.walleij@linaro.org>
To: Stephan Gerhold <stephan@gerhold.net>,
	Ajay Kumar <ajaykumar.rs@samsung.com>,
	Thierry Reding <treding@nvidia.com>
Cc: Neil Armstrong <neil.armstrong@linaro.org>,
	Caleb Connolly <caleb.connolly@linaro.org>,
	linux-arm-msm@vger.kernel.org,
	Joel Selvaraj <joelselvaraj.oss@gmail.com>,
	dri-devel@lists.freedesktop.org,
	~postmarketos/upstreaming@lists.sr.ht,
	Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
	phone-devel@vger.kernel.org, Sam Ravnborg <sam@ravnborg.org>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	Joel Selvaraj <jo@jsfamily.in>
Subject: Re: [PATCH] drm/panel: move some dsi commands from unprepare to disable
Date: Wed, 14 Jun 2023 22:58:56 +0200	[thread overview]
Message-ID: <CACRpkdZ7a3aARMs3iBbBavF_0AkPOPs3fH8e6CrZYo7Ybr6m_A@mail.gmail.com> (raw)
In-Reply-To: <ZIjayn8nVy-ejThH@gerhold.net>

On Tue, Jun 13, 2023 at 11:08 PM Stephan Gerhold <stephan@gerhold.net> wrote:

> I'm still quite confused about what exactly is supposed to be in
> (un)prepare and what in enable/disable. I've seen some related
> discussion every now and then but it's still quite inconsistent across
> different panel drivers... Can someone clarify this?

It is somewhat clarified in commit 45527d435c5e39b6eec4aa0065a562e7cf05d503
that added the callbacks:

Author: Ajay Kumar <ajaykumar.rs@samsung.com>
Date:   Fri Jul 18 02:13:48 2014 +0530

    drm/panel: add .prepare() and .unprepare() functions

    Panels often require an initialization sequence that consists of three
    steps: a) powering up the panel, b) starting transmission of video data
    and c) enabling the panel (e.g. turn on backlight). This is usually
    necessary to avoid visual glitches at the beginning of video data
    transmission.

    Similarly, the shutdown sequence is typically done in three steps as
    well: a) disable the panel (e.g. turn off backlight), b) cease video
    data transmission and c) power down the panel.

    Currently drivers can only implement .enable() and .disable() functions,
    which is not enough to implement the above sequences. This commit adds a
    second pair of functions, .prepare() and .unprepare() to allow more
    fine-grained control over when the above steps are performed.

    Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
    [treding: rewrite changelog, add kerneldoc]
    Signed-off-by: Thierry Reding <treding@nvidia.com>

My interpretation is that .enable/.disable is for enabling/disabling
backlight and well, make things show up on the display, and that
happens quickly.

prepare/unprepare is for everything else setting up/tearing down
the data transmission pipeline.

In the clock subsystem the enable/disable could be called in fastpath
and prepare/unprepare only from slowpath so e.g an IRQ handler
can gate a simple gated clock. This semantic seems to have nothing
to do with the display semantic. :/

Yours,
Linus Walleij