Message-ID: <77aea6a7-8a83-b748-5443-a5303f17ff51@ansari.sh>
Date:   Fri, 20 Jan 2023 17:09:29 +0000
From:   Rayyan Ansari <rayyan@...ari.sh>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org
Cc:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE" 
        <devicetree@...r.kernel.org>, asahi@...ts.linux.dev
Subject: Re: [RFC PATCH] drm/simpledrm: Allow physical width and height
 configuration via DT

On 19/01/2023 10:44, Thomas Zimmermann wrote:
> (cc: devicetree@...r.kernel.org, asahi@...ts.linux.dev)
> 
> Hi,
> 
> thanks for the patch. I already wondered if the DPI value should be 
> configurable in some way.
> 
> Am 18.01.23 um 19:48 schrieb Rayyan Ansari:
>> 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@...0000 {
>>             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.
> 
> Is this something that is already supported by some device, or just a 
> pet project of yours?
> 

Phosh is a mobile environment, developed for use on the Librem 5 phone, 
but it's also packaged by mobile-focused distros such as postmarketOS 
and used on other phones.
https://puri.sm/posts/phosh-overview/

This is my Lumia running Phosh with Firefox open: 
https://wiki.postmarketos.org/images/c/c3/Lumia_735_Phosh.png

>>
>> 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?
> 
> I'd use 32 bits in the DT, just like the other properties.
> 

Noted.

>> - 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.
>> - 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.
>> Thanks,
>> Rayyan.
>> ---
>>   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,
> 
> Maybe call it simplefb_read_u16_of_opt()
> 
>> +             const char *name, u16 *value)
> 
> The alignment looks off.
> 
>> +{
>> +    int ret = of_property_read_u16(of_node, name, value);
>> +    if (ret)
>> +        value = 0;
> 
> You mean *value = 0 ?
> 
> I think we should be stricter here. Look at the docs at [1]. A result of 
> 0 means success and -EINVAL means that the property does not exist. We 
> should still report errors for the other errno codes.
> 
> Something like
> 
>    ret = of_property_read_u16()
> 
>    if (ret) {
>      if(ret == -EINVAL) {
>          *value = 0;
>      ret= 0;
>      } else {
>          drm_err(dev, "simplefb: cannot parse framebuffer %s:
>              "error %d\n", name, ret);
>      }
>    }
> 
>    return ret;
> 
> [1] https://elixir.bootlin.com/linux/latest/source/include/linux/of.h#L1202
> 

Will change, thanks for the example and reference.

>> +}
>> +
>>   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;
>> +}
>>   /*
>>    * 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
> 
> Please move 'dimensions' to the previous line to make it more pleasant 
> to the eyes.
> 
>> +     * 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))
>> +            )
> 
> The coding style is awkward and the ?: doesn't belong here. Please see 
> further below.
> 
>>       };
>>       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;
> 
> Init those two variables to zero.
> 
>>       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);
>>       }
> 
> Just like with the framebuffer stride, here's the place to detect 
> default values. So at this point, do something like
> 
>       if (!width_mm)
>          width_mm = DRM_MODE_RES_MM(width, 96ul);
>       if (!height_mm)
>          height_mm = DRM_MODE_RES_MM(height, 96ul);
> 
> And pass the initialized physical dimensions to simpldrm_mode(). You can 
> move the comment in simpledrm_mode() before the branches.
> 
> Best regards
> Thomas
> 
>> -    sdev->mode = simpledrm_mode(width, height);
>> +    sdev->mode = simpledrm_mode(width, height, width_mm, height_mm);
>>       sdev->format = format;
>>       sdev->pitch = stride;
> 

Thanks for the feedback! I'll improve on this patch and write 
documentation to hopefully make this mainline-ready.

-- 
Rayyan Ansari
https://ansari.sh

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