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

[Aditya Garg <gargaditya08@live.com>]

> On 25 Feb 2025, at 9:36 PM, Thomas Zimmermann <tzimmermann@suse.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.
> 
> 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)
>