Message-ID: <90afab05-eb56-4c81-afd5-35337335c562@suse.de>
Date: Tue, 9 Dec 2025 09:42:03 +0100
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Helge Deller <deller@....de>, Chintan Patel <chintanlike@...il.com>,
 linux-fbdev@...r.kernel.org, linux-staging@...ts.linux.dev,
 linux-omap@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 andy@...nel.org, gregkh@...uxfoundation.org
Subject: Re: [PATCH 0/3] fbdev: Guard sysfs interfaces under CONFIG_FB_DEVICE

Hi

Am 09.12.25 um 09:22 schrieb Helge Deller:
> On 12/9/25 08:27, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 09.12.25 um 05:27 schrieb Chintan Patel:
>>> Hi all,
>>>
>>> This small series makes several legacy fbdev drivers buildable with
>>> CONFIG_FB_DEVICE=n. Currently, multiple fbdev drivers rely on 
>>> fb_info->dev
>>> and sysfs attribute registration unconditionally, which leads to build
>>> failures whenever FB_DEVICE is disabled.
>>>
>>> Thomas previously noted that FB_DEVICE should eventually become 
>>> optional
>>> and that drivers should not depend on sysfs or fb_info->dev being 
>>> present
>>> unless the Kconfig explicitly selects it. This series pushes in that
>>> direction by tightening the FB_DEVICE dependency boundary without 
>>> changing
>>> any runtime behaviour when FB_DEVICE=y.
>>>
>>> What this series does *not* change
>>>
>>> - No functional behaviour changes when FB_DEVICE=y.
>>> - No removal of sysfs interfaces.
>>> - No changes to fbops, memory allocation, or display update paths.
>>>
>>> Build & test coverage
>>>
>>> Tested with the following combinations:
>>>
>>> 1. **FB=y, FB_DEVICE=y**
>>>     - Baseline configuration; no regressions expected.
>>>
>>> 2. **FB=y, FB_DEVICE=n**
>>>     - Drivers build successfully.
>>>     - No sysfs attributes are created.
>>>     - fbdev devices operate normally (where applicable).
>>>
>>> 3. **FB=n**
>>>     - Drivers depend on FB, so they properly do not build, unchanged.
>>>
>>> Motivation
>>>
>>> This moves fbdev closer to supporting FB_DEVICE as truly optional, 
>>> helps
>>> reduce Kconfig entanglement, and clears several long-standing TODO 
>>> items
>>> as suggested by Thomas Zimmermann around legacy sysfs usage inside 
>>> fbdev
>>> drivers.
>>>
>>> Feedback is welcome, especially on whether the guard boundaries around
>>> sysfs are placed correctly or whether more logic should be pulled under
>>> CONFIG_FB_DEVICE.
>>
>> I left a comment on the first patch. If things still build nicely, then
>>
>> Acked-by: Thomas Zimmermann <tzimmermann@...e.de>
>>
>> for the series.
>
> This whole series adds a whole lot of ifdef'ery, which I think is the
> worst approach. It makes the code less readable and leads to two code
> paths, which may trigger different build errors depending on the config.
>
> I'm sure it must be possible to do the same without adding more #ifdefs,
> e.g. by introducing a function like   dev_of_fbinfo(fbinfo)  which
> simply returns NULL for the FB_DEVICE=n case.  Then, that value can be 
> tested
> like
>     if (dev_of_fbinfo(fbinfo))
>         {...do-the-things...}
> For the FB_DEVICE=n case this will then be optimized out by the compiler,
> while you still have full compiler syntax checking.
>
> Thoughts?

Your choice. I don't see this as an important fix. The FB_DEVICE=n case 
is mostly useful for DRM-based systems that do not want to expose fbdev 
interfaces to user space. Those are the vast majority today. The very 
few special builds with fbdev drivers would likely use FB_DEVICE=y anyway.

Best regards
Thomas

>
> Helge
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)

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