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From:   Guillaume Ranquet <granquet@baylibre.com>
User-Agent: alot/0.10
Date:   Wed, 15 Dec 2021 09:03:01 -0600
Message-ID: <CABnWg9uyoK0TkRZRJXstmtB4u2-UUCi-x_frosKhhQerNmFT=A@mail.gmail.com>
Subject: Re: [PATCH v6 7/7] drm/mediatek: Add mt8195 DisplayPort driver
To:     Maxime Ripard <maxime@cerno.tech>
Cc:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Chun-Kuang Hu <chunkuang.hu@kernel.org>,
        Philipp Zabel <p.zabel@pengutronix.de>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Markus Schneider-Pargmann <msp@baylibre.com>,
        kernel test robot <lkp@intel.com>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-mediatek@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org
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Hi Maxime,

Quoting Maxime Ripard (2021-12-13 17:54:22)
> On Thu, Dec 02, 2021 at 06:48:12AM -0800, Guillaume Ranquet wrote:
> > Hi,
> >
> > Quoting Maxime Ripard (2021-11-25 15:30:34)
> > > On Wed, Nov 24, 2021 at 01:45:21PM +0000, Guillaume Ranquet wrote:
> > > > Hi,
> > > > Thanks for all your input, really appreciated.
> > > >
> > > > Quoting Maxime Ripard (2021-11-16 15:51:12)
> > > > > Hi,
> > > > >
> > > > > On Mon, Nov 15, 2021 at 09:33:52AM -0500, Guillaume Ranquet wrote:
> > > > > > Quoting Maxime Ripard (2021-11-15 11:11:29)
> > > > > > > > The driver creates a child device for the phy. The child device will
> > > > > > > > never exist without the parent being active. As they are sharing a
> > > > > > > > register range, the parent passes a regmap pointer to the child so that
> > > > > > > > both can work with the same register range. The phy driver sets device
> > > > > > > > data that is read by the parent to get the phy device that can be used
> > > > > > > > to control the phy properties.
> > > > > > >
> > > > > > > If the PHY is in the same register space than the DP controller, why do
> > > > > > > you need a separate PHY driver in the first place?
> > > > > >
> > > > > > This has been asked by Chun-Kuang Hu in a previous revision of the series:
> > > > > >
> > > > > > https://lore.kernel.org/linux-mediatek/CAAOTY_-+T-wRCH2yw2XSm=ZbaBbqBQ4EqpU2P0TF90gAWQeRsg@mail.gmail.com/
> > > > >
> > > > > It's a bit of a circular argument though :)
> > > > >
> > > > > It's a separate phy driver because it needs to go through another
> > > > > maintainer's tree, but it needs to go through another maintainer's tree
> > > > > because it's a separate phy driver.
> > > > >
> > > > > It doesn't explain why it needs to be a separate phy driver? Why can't
> > > > > the phy setup be done directly in the DP driver, if it's essentially a
> > > > > single device?
> > > > >
> > > > > That being said, usually what those kind of questions mean is that
> > > > > you're missing a comment or something in the commit log to provide that
> > > > > context in the first place, so it would be great to add that context
> > > > > here.
> > > > >
> > > > > And it will avoid the situation we're now in where multiple reviewers
> > > > > ask the same questions over and over again :)
> > > > >
> > > > At first I didn't understand your reply, then I realized I gave you
> > > > the wrong link...
> > > > my bad! I'm struggling a bit with mail reviews, but I'll get there eventually.
> > > >
> > > > The driver and phy were a single driver until v2 of this patch series
> > > > and the phy setup
> > > > was done directly in the driver (single driver, single C file).
> > > > Here's the relevant link to the discussion between Chun-Kuang and Markus
> > > >
> > > > https://lore.kernel.org/linux-mediatek/CAAOTY__cJMqcAieEraJ2sz4gi0Zs-aiNXz38_x7dPQea6HvYEg@mail.gmail.com/#t
> > > >
> > > > I'll try to find a way to make it clearer for v7.
> > >
> > > OK, it makes sense then :)
> > >
> > > There's something weird though: the devices definitely look like they're
> > > in a separate register range, yet you mention a regmap to handle the
> > > shared register range. That range doesn't seem described anywhere in the
> > > device tree though? What is it for?
> >
> > My understanding is that 0x1000 to 0x1fff controls the phy
> > functionalities and 0x2000 to 0x4fff controls "non-phy"
> > functionalities. And you are right, there's no description of that in
> > the device tree whatsoever. The ranges are in the same actual device
> > and thus it has been decided to not have dt-bindings for the phy
> > device.
>
> Sure, that last part makes sense, but then I'm not sure why you don't
> have the full register range in the device node you have in the DT?
>
> > The phy driver is a child of the DP driver that we register using
> > platform_device_register_data() and we pass along the same regmap as
> > the DP driver in its platform data.
>
> Especially if it's used by something, it should be described in the DT
> somewhere.
>
> Maxime


So, to make things crystal clear to a newbie (like me).
Would you describe it like this:
compatible = "mediatek,mt8195-dp-tx";
reg = <0 0x1c501000 0 0x0fff>,
	<0 0x1c502000 0 0x2fff>;

instead of the current description:
compatible = "mediatek,mt8195-dp-tx";
reg = <0 0x1c500000 0 0x8000>;

I haven't checked what the rest of the 0x8000 range is used for though...

Thx,
Guillaume.