linux-kernel - Re: [PATCH 3/5] gpu: drm: adp: Add a backlight driver for the Summit LCD

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Message-ID: <e647e8c7-6df9-44f5-abcc-34db74b8e266@kernel.org>
Date: Mon, 25 Nov 2024 16:06:54 +0100
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Nick Chan <towinchenmi@...il.com>, fnkl.kernel@...il.com,
 Hector Martin <marcan@...can.st>, Sven Peter <sven@...npeter.dev>,
 Alyssa Rosenzweig <alyssa@...enzweig.io>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>,
 Neil Armstrong <neil.armstrong@...aro.org>,
 Jessica Zhang <quic_jesszhan@...cinc.com>, asahi@...ts.linux.dev
Cc: linux-arm-kernel@...ts.infradead.org, dri-devel@...ts.freedesktop.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] gpu: drm: adp: Add a backlight driver for the Summit
 LCD

On 25/11/2024 16:03, Nick Chan wrote:
>>> +static int summit_probe(struct mipi_dsi_device *dsi)
>>> +{
>>> +	struct backlight_properties props = { 0 };
>>> +	struct device *dev = &dsi->dev;
>>> +	struct summit_data *panel;
>>> +
>>> +	panel = devm_kzalloc(dev, sizeof(*panel), GFP_KERNEL);
>>> +	if (!panel)
>>> +		return -ENOMEM;
>>> +
>>> +	mipi_dsi_set_drvdata(dsi, panel);
>>> +	panel->dsi = dsi;
>>> +
>>> +	int ret = device_property_read_u32(dev, "max-brightness", &props.max_brightness);
>> That's an undocumented property, which suggests you did not test your DTS.
> 
> Actually, testing the DTS would not have caught this issue. For more
> context,

If you mean that property is not in your DTS, then right, it would not
be caught. But otherwise it would.

Anyway, I pointed out testing as helping tool for your development, just
like building with clang W=1 will spare you some review comments or bugs.

> all summit panels found in touch bar have a max brightness of 255, but the
> summit panel in Apple A11 devices like the iPhone X is latter found to have
> a max brightness of 2047.
> 
> However, A11 cannot be properly supported right now due to not having a
> driver
> for the DART IOMMU.
> 
> In the meantime, max-brightness could documented and be made required,
> and the
> default 255 brightness could be removed.

BTW, max-brightness is a property of backlight, not panel, I think.

Best regards,
Krzysztof