drm fb helpers hotplug/resize

[Ville Syrjälä]

On Fri, Oct 07, 2022 at 09:58:31AM +0200, Thomas Zimmermann wrote:
> 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]

Uff. Right, we'd probably just want to properly implemnt set_par()
then. 

BTW I had a slightly different take on the bitfiled stuff.
Maybe a bit easier to extend for new formats, but full of macros:
https://patchwork.freedesktop.org/patch/203189/?series=37820&rev=1

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

I guess we could do a TEST_ONLY commit there. Though I think
not doing that and just failing from set_par() should be fine
too.

> 
> 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)
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-- 
Ville Syrjälä
Intel