drm fb helpers hotplug/resize

[Thomas Zimmermann]

Hi

Am 07.10.22 um 09:15 schrieb Ville Syrjälä:

>>
>> For the absolute size of fbdev memory, I think we should introduce a
>> module parameter in drm_fb_helper, which an option to set a default
>> value in the kernel config. It would benefit all drivers that use fbdev
>> emulation and work how overalloc works.
>>
>> If no size is given, the current approach would be used.
>>
>> I don't think resizing would work immediately. There isn't anything in
>> the check_var and set_par functions that implements the necessary atomic
>> check and commit.
> 
> set_par() is the thing tht does the commit.
> 

I know. There actually even is a commit statement in set_par, which can 
restore the initial mode. My point is that it has no means of changing 
the display mode. A full modeset would require us to convert the fb 
screeninfo into an atomic state and commit that instead. For the fbconv 
helper, I once made code to convert between the two. Leaving this here 
for reference. [1][2]

Similarly, in check_var we sort out and reject all mode changes. We'd 
have to change that as well.

I guess we can continue to ignore non-atomic modesetting.

Best regards
Thomas

[1] 
https://gitlab.freedesktop.org/tzimmermann/linux/-/commit/385161cd2d048b5cf80544bff8ced3da7a82dfa9
[2] 
https://gitlab.freedesktop.org/tzimmermann/linux/-/commit/a541c405a638f47ee80389b222fbde6e311e8220

-- 
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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