[PATCH 2/2] fbdev: Add support for the nomodeset kernel parameter

[Thomas Zimmermann]

Hi

Am 07.11.22 um 14:57 schrieb Helge Deller:
> On 11/7/22 11:49, Thomas Zimmermann wrote:
>> Support the kernel's nomodeset parameter for all PCI-based fbdev
>> drivers that use aperture helpers to remove other, hardware-agnostic
>> graphics drivers.
>>
>> The parameter is a simple way of using the firmware-provided scanout
>> buffer if the hardware's native driver is broken.
> 
> Nah... it's probably not broken, but you want it disabled in order
> to use the DRM driver instead?

No, it's really for broken native drivers or any kind of problematic 
modesetting. Most DRM drivers already respect the nomodeset option and 
won't load when given. All you'd get are the generic drivers, such as 
simpledrm, efifb or simplefb.

There are better options of configuring video output on the kernel 
command line.  But as graphics output is such a fundamental feature to 
using a computer, we found that a simple and easy option to workaround 
erroneous systems would benefit DRM users; hence the nomodeset parameter.

As fbdev drivers also do modesetting, supporting the parameter simply 
unifies the behavior.

> 
>> The same effect
>> could be achieved with per-driver options, but the importance of the
>> graphics output for many users makes a single, unified approach
>> worthwhile.
>>
>> With nomodeset specified, the fbdev driver module will not load. This
>> unifies behavior with similar DRM drivers. In DRM helpers, modules
>> first check the nomodeset parameter before registering the PCI
>> driver. As fbdev has no such module helpers, we have to modify each
>> driver individually.
> 
> Ok.
> 
>> The name 'nomodeset' is slightly misleading, but has been chosen for
>> historical reasons. Several drivers implemented it before it became a
>> general option for DRM. So keeping the existing name was preferred over
>> introducing a new one.
> 
>> diff --git a/drivers/video/fbdev/aty/aty128fb.c 
>> b/drivers/video/fbdev/aty/aty128fb.c
>> index 57e398fe7a81c..1a26ac2865d65 100644
>> --- a/drivers/video/fbdev/aty/aty128fb.c
>> +++ b/drivers/video/fbdev/aty/aty128fb.c
>> @@ -2503,7 +2504,12 @@ static int aty128fb_init(void)
>>   {
>>   #ifndef MODULE
>>       char *option = NULL;
>> +#endif
>> +
>> +    if (video_firmware_drivers_only())
>> +        return -ENODEV;
> 
> I think it makes sense to give at least some info, why a specific
> driver wasn't loaded, e.g. something like this kernel message:
> aty128fb: Driver disabled due to "nomodeset" kernel parameter.
> 
> If you e.g. change the function video_firmware_drivers_only()
> to become video_firmware_drivers_only(const char *drivername)
> then you could print such a message in video_firmware_drivers_only()

Well, we do have such a message in disable_modeset() already. [1]

[1] 
https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_nomodeset.c#L18

> 
> And I don't like very much the name of function 
> video_firmware_drivers_only(),
> but don't have any other better idea right now either...

It's part of the 'video' module, hence the prefix. The 'nomodeset' 
option used to be implemented in several DRM drivers. It's an awful 
name, but we didn't want to remove it or introduce a new one for the 
same feature. So we kept nomodeset for all of DRM.  Then we started 
bikeshedding the function name that returns the setting. And 
firmware-drivers-only is the best description of what is happening here. 
So that's how the name happend.

Best regards
Thomas

> 
> Helge

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev
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