Re: [PATCH v5 2/2] drm/tiny: add driver for Apple Touch Bars in x86 Macs

["andriy.shevchenko@linux.intel.com" <andriy.shevchenko@linux.intel.com>]

On Tue, Feb 25, 2025 at 10:09:42AM +0000, Aditya Garg wrote:
> From: Kerem Karabay <kekrby@gmail.com>
> 
> The Touch Bars found on x86 Macs support two USB configurations: one
> where the device presents itself as a HID keyboard and can display
> predefined sets of keys, and one where the operating system has full
> control over what is displayed.
> 
> This commit adds support for the display functionality of the second
> configuration. Functionality for the first configuration has been
> merged in the HID tree.
> 
> Note that this driver has only been tested on T2 Macs, and only includes
> the USB device ID for these devices. Testing on T1 Macs would be
> appreciated.
> 
> Credit goes to Ben (Bingxing) Wang on GitHub for reverse engineering
> most of the protocol.
> 
> Also, as requested by Andy, I would like to clarify the use of __packed
> structs in this driver:
> 
> - All the packed structs are aligned except for appletbdrm_msg_information.
> - We have to pack appletbdrm_msg_information since it is requirement of
>   the protocol.
> - We compared binaries compiled by keeping the rest structs __packed and
>   not __packed using bloat-o-meter, and __packed was not affecting code
>   generation.
> - To maintain consistency, rest structs have been kept __packed.
> 
> I would also like to point out that since the driver was reverse-engineered
> the actual data types of the protocol might be different, including, but
> not limited to, endianness.

...

> +static int appletbdrm_probe(struct usb_interface *intf,
> +			    const struct usb_device_id *id)
> +{
> +	struct usb_endpoint_descriptor *bulk_in, *bulk_out;
> +	struct device *dev = &intf->dev;
> +	struct appletbdrm_device *adev;
> +	struct drm_device *drm;
> +	int ret;
> +
> +	ret = usb_find_common_endpoints(intf->cur_altsetting, &bulk_in, &bulk_out, NULL, NULL);
> +	if (ret) {
> +		drm_err(drm, "Failed to find bulk endpoints\n");

This is simply wrong (and in this case even lead to crash in some circumstances).
drm_err() may not be used here. That's my point in previous discussions.
Independently on the subsystem the ->probe() for the sake of consistency and
being informative should only rely on struct device *dev,

> +		return ret;
> +	}
> +
> +	adev = devm_drm_dev_alloc(dev, &appletbdrm_drm_driver, struct appletbdrm_device, drm);
> +	if (IS_ERR(adev))
> +		return PTR_ERR(adev);
> +
> +	adev->in_ep = bulk_in->bEndpointAddress;
> +	adev->out_ep = bulk_out->bEndpointAddress;
> +	adev->dmadev = dev;
> +
> +	drm = &adev->drm;
> +
> +	usb_set_intfdata(intf, adev);
> +
> +	ret = appletbdrm_get_information(adev);
> +	if (ret) {
> +		drm_err(drm, "Failed to get display information\n");
> +		return ret;
> +	}
> +
> +	ret = appletbdrm_signal_readiness(adev);
> +	if (ret) {
> +		drm_err(drm, "Failed to signal readiness\n");
> +		return ret;
> +	}
> +
> +	ret = appletbdrm_setup_mode_config(adev);
> +	if (ret) {
> +		drm_err(drm, "Failed to setup mode config\n");
> +		return ret;
> +	}
> +
> +	ret = drm_dev_register(drm, 0);
> +	if (ret) {
> +		drm_err(drm, "Failed to register DRM device\n");
> +		return ret;
> +	}
> +
> +	ret = appletbdrm_clear_display(adev);
> +	if (ret) {
> +		drm_err(drm, "Failed to clear display\n");
> +		return ret;
> +	}
> +
> +	return 0;
> +}

-- 
With Best Regards,
Andy Shevchenko