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

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Message-ID: <5C84C111-B1F1-4B6C-9671-559F6E1BDB78@live.com>
Date: Tue, 25 Feb 2025 16:40:54 +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 9:39 PM, Aditya Garg <gargaditya08@...e.com> wrote:
> 
> 
> 
>> On 25 Feb 2025, at 9:36 PM, Thomas Zimmermann <tzimmermann@...e.de> wrote:
>> 
>> Hi
>> 
>> Am 25.02.25 um 15:54 schrieb Aditya Garg:
>> [...]
>>>>>> +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.
>> 
>> Maybe start reading those compiler warnings. They are there for a reason. Your compiler tells you that you are dereferencing an uninitialized pointer, right here:
> 
> The thing is, that I didn't get any compiler warnings, even with sparse.

Even though I really didn't get any compiler warnings, and the drm_err was actuality working with drm uninitialised (it just showed NULL in place of appletbdrm), I’ve still cleared it to NULL and sent a v6:

https://lore.kernel.org/dri-devel/0BAB26BE-5BB6-467F-BED3-9AA8DE25433B@live.com/T/#t


>> 
>> https://elixir.bootlin.com/linux/v6.13.4/source/include/drm/drm_print.h#L586
>> 
>> Clearing that pointer to NULL will fix the error and make drm_err() work.
>> 
>> Best regards
>> Thomas
>> 
>>>> 
>>>>>> +        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)
>>>> 
>> 
>> --
>> --
>> 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)