Message-ID:
 <PN3PR01MB9597A550BF7FE762ED5FD172B8C32@PN3PR01MB9597.INDPRD01.PROD.OUTLOOK.COM>
Date: Tue, 25 Feb 2025 14:56:37 +0000
From: Aditya Garg <gargaditya08@...e.com>
To: Thomas Zimmermann <tzimmermann@...e.de>
CC: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>,
	"maarten.lankhorst@...ux.intel.com" <maarten.lankhorst@...ux.intel.com>,
	"mripard@...nel.org" <mripard@...nel.org>, "airlied@...il.com"
	<airlied@...il.com>, "simona@...ll.ch" <simona@...ll.ch>, Kerem Karabay
	<kekrby@...il.com>, Atharva Tiwari <evepolonium@...il.com>, Aun-Ali Zaidi
	<admin@...eit.net>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v5 2/2] drm/tiny: add driver for Apple Touch Bars in x86
 Macs



> On 25 Feb 2025, at 8:24 PM, Aditya Garg <gargaditya08@...e.com> wrote:
> 
> 
> 
>> On 25 Feb 2025, at 5:28 PM, Thomas Zimmermann <tzimmermann@...e.de> wrote:
>> 
>> Hi
>> 
>>> Am 25.02.25 um 11:33 schrieb andriy.shevchenko@...ux.intel.com:
>>>>> On Tue, Feb 25, 2025 at 10:09:42AM +0000, Aditya Garg wrote:
>>>>> From: Kerem Karabay <kekrby@...il.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,
>> 
>> That's never going to work with DRM. There's so much code in a DRM probe function that uses the DRM error functions.
>> 
>> This specific instance here is wrong, as the drm pointer hasn't been initialized. But as soon as it is, it's much better to use drm_err() and friends. It will do the right thing and give consistent output across drivers.
>> 
> Ok so this is actually an interesting case, since I am trying to fix it. To initialise the drm pointer, we need to initialise adev, and to initialise adev, we need to run usb_find_common_endpoints first. So IMO, we cannot use drm_err here, but rather dev_err_probe can be used.

Infact dev is also not initialised......
>> 
>> 
>>> 
>>>> +        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;
>>>> +}
>> 
>> --
>> --
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Frankenstrasse 146, 90461 Nuernberg, Germany
>> GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
>> HRB 36809 (AG Nuernberg)
>> 

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