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

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Message-ID: <71123020-f345-4de3-9044-ad58f8066d86@suse.de>
Date: Tue, 25 Feb 2025 14:53:15 +0100
From: Thomas Zimmermann <tzimmermann@...e.de>
To: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>
Cc: Aditya Garg <gargaditya08@...e.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

Hi

Am 25.02.25 um 14:27 schrieb andriy.shevchenko@...ux.intel.com:
> On Tue, Feb 25, 2025 at 12:59:43PM +0100, Thomas Zimmermann wrote:
>> Am 25.02.25 um 12:01 schrieb andriy.shevchenko@...ux.intel.com:
>>> On Tue, Feb 25, 2025 at 10:48:53AM +0000, Aditya Garg wrote:
>>>>> On 25 Feb 2025, at 4:17 PM, andriy.shevchenko@...ux.intel.com wrote:
>>>>> On Tue, Feb 25, 2025 at 10:36:03AM +0000, Aditya Garg wrote:
>>>>>>>> On 25 Feb 2025, at 4:03 PM, andriy.shevchenko@...ux.intel.com wrote:
>>>>>>> On Tue, Feb 25, 2025 at 10:09:42AM +0000, Aditya Garg wrote:
>   ...
>
>>>>>>>> +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,
>>>>>> I'm not sure how drm_err works,
>>>>> It's a macro.
>>>>>
>>>>>> but struct drm_device does have a struct device *dev as well.
>>>>> Yes, but only when it's initialized.
>>>>>
>>>>>> Anyways, this is something I'll leave for Thomas to reply.
>>>>> The code above is wrong independently on his reply :-)
>>>> I'm kinda stuck between contrasting views of 2 kernel maintainers lol,
>>>> so I said let Thomas reply.
>>> Sure. I also want him to clarify my question about potential drm_err_probe().
>> These threads get a little lengthy. What is the question?
> How drm_err_probe() can be (consistently) implemented as there are and will be
> cases when we want to return an error code with the message and having DRM devce
> not being available, please?

The DRM logging works with a DRM device pointer of NULL. It'll simply 
leave out device infos.

>
> Also, drm_err() has a downside of not checking for deferred probe and
> potentially leads to the noisy log floods.

I think it should be possible to export __dev_probe_failed() [1] from 
the core and write drm_err_probe() and drm_warn_probe() around this. The 
output then looks like a DRM logging, but behaves like dev-based 
logging. Note that DRM logging already is an elaborate wrapper around 
the dev-based logging, so it will be more of the same.

Best regards
Thomas

[1] 
https://elixir.bootlin.com/linux/v6.13.4/source/drivers/base/core.c#L5008

>

-- 
--
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)