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Date:   Thu, 19 Jan 2023 11:01:04 +0100
From:   Janne Grunau <janne@jannau.net>
To:     Rayyan Ansari <rayyan@ansari.sh>
Cc:     dri-devel@lists.freedesktop.org,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Javier Martinez Canillas <javierm@redhat.com>,
        linux-kernel@vger.kernel.org, devicetree@vger.kernel.org,
        asahi@lists.linux.dev
Subject: Re: [RFC PATCH] drm/simpledrm: Allow physical width and height
 configuration via DT
Message-ID: <20230119100104.GE3576@jannau.net>
References: <20230118184817.608551-1-rayyan@ansari.sh>
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Hej,

adding devicetree@vger.kernel.org and asahi@lists.linux.dev to cc:, the 
former for the obvious devictree/bindings related questions,
asahi for the prospect of supporting high DPI displays during early boot 
and in u-boot.

On 2023-01-18 18:48:17 +0000, Rayyan Ansari wrote:
> Hello,
> The following draft patch adds support for configuring the
> height-mm and width-mm DRM properties in the simpledrm driver
> via devicetree.
> This is useful to get proper scaling in UIs such as Phosh.
> An example of using this property is this, taken from my local tree:
> 
> 		framebuffer0: framebuffer@3200000 {
> 			compatible = "simple-framebuffer";
> 			reg = <0x3200000 0x800000>;
> 			format = "a8r8g8b8";
> 			width = <720>;
> 			height = <1280>;
> 			stride = <(720 * 4)>;
> 			width-mm = /bits/ 16 <58>;
> 			height-mm = /bits/ 16 <103>;
> 
> 			clocks = <&mmcc MDSS_AHB_CLK>,
> 				 <&mmcc MDSS_AXI_CLK>,
> 				 <&mmcc MDSS_BYTE0_CLK>,
> 				 <&mmcc MDSS_MDP_CLK>,
> 				 <&mmcc MDSS_PCLK0_CLK>,
> 				 <&mmcc MDSS_VSYNC_CLK>;
> 			power-domains = <&mmcc MDSS_GDSC>;
> 		};
> 
> I have tested this on my Lumia 735, and it does indeed
> allow Phosh to scale correctly on the screen.
> 
> However, I would like to get some feedback before I write the
> documentation.
> - What data type should be used?
> 	The width_mm and height_mm properties of the drm_display_mode
> 	struct are defined as u16. I have also made the devicetree
> 	properties as the u16 type, but this requires specifying
> 	"/bits/ 16" before the value. Should u32 be used instead to get
> 	rid of this? If so, how could the conversion from u32->u16 be
> 	handled?

u32 is the appropriate type. The device tree describes the hardware and 
not the data types used in a "random" linux driver/subsystem. 65m is 
probably enough for all practical purposes but u32 is the better choice.  
Documentation/devicetree/bindings/display/panel/panel-common.yaml 
already specifies "height-mm" and "width-mm" and all device tree files 
using this binding code the properties as u32.

We probably do not want add height and width properties to the 
simple-framebuffer node directly. At least for the static case I would 
expect that it duplicates information already present in a panel node.  
For that case parsing the panel dimensions via a phandle reference to 
that panel node would be preferred.

I'm not sure if it worth considering the dynamic case. The bootloader 
may be able to provide dimensions of HDMI, DP, ...  connected displays 
from the EDID. In that case "height-mm" and "width-mm" properties would 
make sense.

The existing panel drivers seem to ignore the u32 -> u16 conversion 
problem.

> - Style?
> 	I have split the arguments to the DRM_MODE_INIT macro across
> 	multiple lines to increase readability. I'm not sure if this
> 	is the correct style though.

I think the code would be more readable if width_mm and height_mm would 
be calculated outside of DRM_MODE_INIT if they are zero.

> - Anything else?
> 	This is my first time writing code for a Linux driver, so I
> 	would be grateful if you have any suggestions for improvements.

Documentation/devicetree/bindings/display/simple-framebuffer.yaml needs 
to be updates to list and document the properties added to the node.

> ---
>  drivers/gpu/drm/tiny/simpledrm.c | 49 +++++++++++++++++++++++++++-----
>  1 file changed, 42 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/tiny/simpledrm.c b/drivers/gpu/drm/tiny/simpledrm.c
> index 162eb44dcba8..92109f870b35 100644
> --- a/drivers/gpu/drm/tiny/simpledrm.c
> +++ b/drivers/gpu/drm/tiny/simpledrm.c
> @@ -116,6 +116,15 @@ simplefb_get_format_pd(struct drm_device *dev,
>  	return simplefb_get_validated_format(dev, pd->format);
>  }
>  
> +static void
> +simplefb_read_u16_of_optional(struct drm_device *dev, struct device_node *of_node,
> +		     const char *name, u16 *value)
> +{
> +	int ret = of_property_read_u16(of_node, name, value);
> +	if (ret)
> +		value = 0;
> +}
> +
>  static int
>  simplefb_read_u32_of(struct drm_device *dev, struct device_node *of_node,
>  		     const char *name, u32 *value)
> @@ -184,6 +193,21 @@ simplefb_get_format_of(struct drm_device *dev, struct device_node *of_node)
>  	return simplefb_get_validated_format(dev, format);
>  }
>  
> +static u16
> +simplefb_get_width_mm_of(struct drm_device *dev, struct device_node *of_node)
> +{
> +	u16 width_mm;
> +	simplefb_read_u16_of_optional(dev, of_node, "width-mm", &width_mm);
> +	return width_mm;
> +}
> +
> +static u16
> +simplefb_get_height_mm_of(struct drm_device *dev, struct device_node *of_node)
> +{
> +	u16 height_mm;
> +	simplefb_read_u16_of_optional(dev, of_node, "height-mm", &height_mm);
> +	return height_mm;
> +}

I don't think it makes sense to have these two mostly identical wrapper 
functions. Please pass the name of the property as parameter. It could 
make sense to have a function to both height and width. I think we 
should ignore both height and width if one fails to parse or is 0.
That could of course also be done in simpledrm_mode() for example like:

|	if (!width_mm || !height_mm) {
|		width_mm = DRM_MODE_RES_MM(width, 96ul);
|		height_mm = DRM_MODE_RES_MM(height, 96ul);
|	}

>  /*
>   * Simple Framebuffer device
>   */
> @@ -599,16 +623,24 @@ static const struct drm_mode_config_funcs simpledrm_mode_config_funcs = {
>   */
>  
>  static struct drm_display_mode simpledrm_mode(unsigned int width,
> -					      unsigned int height)
> +					      unsigned int height,
> +					      u16 width_mm,
> +					      u16 height_mm)
>  {
>  	/*
> -	 * Assume a monitor resolution of 96 dpi to
> -	 * get a somewhat reasonable screen size.
> +	 * Assume a monitor resolution of 96 dpi if physical
> +	 * dimensions are not specified to get a somewhat reasonable
> +	 * screen size.
>  	 */
> +
>  	const struct drm_display_mode mode = {
> -		DRM_MODE_INIT(60, width, height,
> -			      DRM_MODE_RES_MM(width, 96ul),
> -			      DRM_MODE_RES_MM(height, 96ul))
> +		DRM_MODE_INIT(
> +			60,
> +			width,
> +			height,
> +			(width_mm ? width_mm : DRM_MODE_RES_MM(width, 96ul)),
> +			(height_mm ? height_mm : DRM_MODE_RES_MM(height, 96ul))
> +			)
>  	};
>  
>  	return mode;
> @@ -622,6 +654,7 @@ static struct simpledrm_device *simpledrm_device_create(struct drm_driver *drv,
>  	struct simpledrm_device *sdev;
>  	struct drm_device *dev;
>  	int width, height, stride;
> +	u16 width_mm, height_mm;

these need to be initialized to 0 otherwise they may end up used 
unitialized if pd is not NULL.

>  	const struct drm_format_info *format;
>  	struct resource *res, *mem;
>  	void __iomem *screen_base;
> @@ -676,6 +709,8 @@ static struct simpledrm_device *simpledrm_device_create(struct drm_driver *drv,
>  		format = simplefb_get_format_of(dev, of_node);
>  		if (IS_ERR(format))
>  			return ERR_CAST(format);
> +		width_mm = simplefb_get_width_mm_of(dev, of_node);
> +		height_mm = simplefb_get_height_mm_of(dev, of_node);
>  	} else {
>  		drm_err(dev, "no simplefb configuration found\n");
>  		return ERR_PTR(-ENODEV);
> @@ -686,7 +721,7 @@ static struct simpledrm_device *simpledrm_device_create(struct drm_driver *drv,
>  			return ERR_PTR(-EINVAL);
>  	}
>  
> -	sdev->mode = simpledrm_mode(width, height);
> +	sdev->mode = simpledrm_mode(width, height, width_mm, height_mm);
>  	sdev->format = format;
>  	sdev->pitch = stride;

Janne