Re: [Linux-fbdev-devel] drm_fb_helper: Impossible to change video mode

[Alex Deucher <alexdeucher@gmail.com>]

On Wed, Mar 10, 2010 at 12:42 PM, James Simmons <jsimmons@infradead.org> wrote:
>
>> >> At the moment the problem with fbset is what to do with it in the
>> >> dual head case. Currently we create an fb console that is lowest
>> >> common size of the two heads and set native modes on both,
>> >
>> > Does that mean that fbset is supposed to work (set resolution) on drmfb?
>>
>> No we've never hooked it up but it could be made work.
>
> I had it to the point of almost working. I plan on working on getting it
> working again.
>
>> > Schemes which would make a multihead setup look like a single screen
>> > get complicated quite easily. Perhaps an option to turn off some
>> > outputs so that the native resolution of one output is used (instead
>> > of clone) would work.
>> >
>>
>> I've only really got two answer for this:
>>
>> (a) hook up another /dev/dri/card_fb device and use the current KMS
>> ioctls to control the framebuffer, have the drm callback into fbdev/fbcon
>> to mention resizes etc. Or add one or two info gathering ioctls and
>> allow use of the /dev/dri/control device to control stuff.
>>
>> (b) add a lot of ioctls to KMS fbdev device, which implement some sort
>> of sane multi-output settings.
>>
>> Now the second sounds like a lot of work if not the correct solution,
>> you basically needs a way to pretty much expose what the KMS ioctls
>> expose on the fb device, and then upgrade fbset to make sense of it all.
>
> Yuck. See my other post about what fbdev really means in its historical
> context. The struct fb_info really maps better to drm_crtc than to
> drm_framebuffer. In fact take the case of the matrox fbdev driver. It
> creates two framebuffer devices even tho it used one static framebuffer.
> What the driver does is splits the framebuffer in two and assigned each
> part to a CRTC.

The only problem with that is that it eats a lot of memory for the
console which limits X when it starts. On cards with limited vram ,
you might not have enough memory left for any meaningful acceleration
when X starts.

Alex

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