Re: [PATCH v3 1/1] backlight: hid_bl: Add VESA VCP HID backlight driver

[Christophe JAILLET]

Le 20/08/2023 à 11:41, Julius Zint a écrit :
> The HID spec defines the following Usage IDs (p. 345 ff):
>
> - Monitor Page (0x80) -> Monitor Control (0x01)
> - VESA Virtual Controls Page (0x82) -> Brightness (0x10)
>
> Apple made use of them in their Apple Studio Display and most likely on
> other external displays (LG UltraFine 5k, Pro Display XDR).
>
> The driver will work for any HID device with a report, where the
> application matches the Monitor Control Usage ID and:
>
> 1. An Input field in this report with the Brightness Usage ID (to get
>     the current brightness)
> 2. A Feature field in this report with the Brightness Usage ID (to
>     set the current brightness)
>
> This driver has been developed and tested with the Apple Studio Display.
> Here is a small excerpt from the decoded HID descriptor showing the
> feature field for setting the brightness:
>
>    Usage Page (Monitor VESA VCP),  ; Monitor VESA VPC (82h, monitor page)
>    Usage (10h, Brightness),
>    Logical Minimum (400),
>    Logical Maximum (60000),
>    Unit (Centimeter^-2 * Candela),
>    Unit Exponent (14),
>    Report Size (32),
>    Report Count (1),
>    Feature (Variable, Null State),
>
> The full HID descriptor dump is available as a comment in the source
> code.
>
> Signed-off-by: Julius Zint <julius@xxxxxxx>
> ---

[...]

> +static void hid_bl_remove(struct hid_device *hdev)
> +{
> +	struct backlight_device *bl;
> +	struct hid_bl_data *data;
> +
> +	hid_dbg(hdev, "remove\n");
> +	bl = hid_get_drvdata(hdev);
> +	data = bl_get_data(bl);
> +
> +	devm_backlight_device_unregister(&hdev->dev, bl);

Hi,

If this need to be called here, before the hid_() calls below, there 
seems to be no real point in using devm_backlight_device_register() / 
devm_backlight_device_unregister().

The non-devm_ version should be enough.

> +	hid_hw_close(hdev);
> +	hid_hw_stop(hdev);
> +	hid_set_drvdata(hdev, NULL);
> +	devm_kfree(&hdev->dev, data);

'data' is devm_kzalloc()'ed.
It is likely that this explicit devm_kfree() could be saved. It will be 
freed by the framework.

> +}

[...]

> +static int hid_bl_probe(struct hid_device *hdev, const struct hid_device_id *id)
> +{
> +	int ret;
> +	struct hid_field *input_field;
> +	struct hid_field *feature_field;
> +	struct hid_bl_data *data;
> +	struct backlight_properties props;
> +	struct backlight_device *bl;
> +
> +	hid_dbg(hdev, "probe\n");
> +
> +	ret = hid_parse(hdev);
> +	if (ret) {
> +		hid_err(hdev, "parse failed: %d\n", ret);
> +		return ret;
> +	}
> +
> +	ret = hid_hw_start(hdev, HID_CONNECT_DRIVER);
> +	if (ret) {
> +		hid_err(hdev, "hw start failed: %d\n", ret);
> +		return ret;
> +	}
> +
> +	input_field = hid_get_usage_field(hdev, HID_INPUT_REPORT,
> +					  HID_USAGE_MONITOR_CTRL,
> +					  HID_USAGE_VESA_VCP_BRIGHTNESS);
> +	if (input_field == NULL) {
> +		ret = -ENODEV;
> +		goto exit_stop;
> +	}
> +
> +	feature_field = hid_get_usage_field(hdev, HID_FEATURE_REPORT,
> +					    HID_USAGE_MONITOR_CTRL,
> +					    HID_USAGE_VESA_VCP_BRIGHTNESS);
> +	if (feature_field == NULL) {
> +		ret = -ENODEV;
> +		goto exit_stop;
> +	}
> +
> +	if (input_field->logical_minimum != feature_field->logical_minimum) {
> +		hid_warn(hdev, "minimums do not match: %d / %d\n",
> +			 input_field->logical_minimum,
> +			 feature_field->logical_minimum);
> +		ret = -ENODEV;
> +		goto exit_stop;
> +	}
> +
> +	if (input_field->logical_maximum != feature_field->logical_maximum) {
> +		hid_warn(hdev, "maximums do not match: %d / %d\n",
> +			 input_field->logical_maximum,
> +			 feature_field->logical_maximum);
> +		ret = -ENODEV;
> +		goto exit_stop;
> +	}
> +
> +	hid_dbg(hdev, "Monitor VESA VCP with brightness control\n");
> +
> +	ret = hid_hw_open(hdev);
> +	if (ret) {
> +		hid_err(hdev, "hw open failed: %d\n", ret);
> +		goto exit_stop;
> +	}
> +
> +	data = devm_kzalloc(&hdev->dev, sizeof(*data), GFP_KERNEL);
> +	if (data == NULL) {
> +		ret = -ENOMEM;
> +		goto exit_stop;

exit_free?
in order to undo the hid_hw_open() just above.

> +	}
> +	data->hdev = hdev;
> +	data->min_brightness = input_field->logical_minimum;
> +	data->max_brightness = input_field->logical_maximum;
> +	data->input_field = input_field;
> +	data->feature_field = feature_field;
> +
> +	memset(&props, 0, sizeof(props));
> +	props.type = BACKLIGHT_RAW;
> +	props.max_brightness = data->max_brightness - data->min_brightness;
> +
> +	bl = devm_backlight_device_register(&hdev->dev, "vesa_vcp",
> +					    &hdev->dev, data,
> +					    &hid_bl_ops,
> +					    &props);
> +	if (IS_ERR(bl)) {
> +		ret = PTR_ERR(bl);
> +		hid_err(hdev, "failed to register backlight: %d\n", ret);
> +		goto exit_free;
> +	}
> +
> +	hid_set_drvdata(hdev, bl);
> +
> +	return 0;
> +
> +exit_free:
> +	hid_hw_close(hdev);
> +	devm_kfree(&hdev->dev, data);

'data' is devm_kzalloc()'ed.
It is likely that this explicit devm_kfree() could be saved. It will be 
freed by the framework.

> +
> +exit_stop:
> +	hid_hw_stop(hdev);
> +	return ret;
> +}

[...]