[RFC] How to add hardware rotation, scaling etc to a DRM/KMS driver

[Daniel Vetter]

On Wed, Jan 19, 2022 at 05:55:22PM +0900, Daniel Palmer wrote:
> Hi all,
> 
> I've copied and pasted my way to mostly working DRM/KMS driver for a
> low cost ARM SoC (Sigmastar SSD202D). The hardware is 2D only.
> 
> One of the devices that uses this SoC has the screen upside down so it
> needs the screen rotated.
> The hardware doesn't have bits that change the scan out direction from
> what I can tell (so it can't mirror/flip while feeding it to the
> screen) but it does have a 2D blitter style block that can take a
> framebuffer and flip/mirror/scale/convert the colour space into
> another buffer.
> 
> My idea was to create a buffer for the rotated image when allocating
> the framebuffer and trigger the hardware to do the conversion each
> vblank or something.
> 
> While reading the discussion about maintaining fbdev I realised maybe
> I should ask instead of wasting too much time on something that's
> wrong.
> 
> I got the feeling that maybe I should just provide an interface to the
> blitter from userspace and userspace should be doing the rotation. I'd
> like to do it in the kernel so stuff like SDL1 apps just work but
> maybe that isn't possible?

panel orientation property is for that stuff:

https://dri.freedesktop.org/docs/drm/gpu/drm-kms.html#standard-connector-properties

You need to scroll down to the "panel orientation:" property. And here's
how to quirk this for at least some cases (for dt panels it should be in
the dt):

https://dri.freedesktop.org/docs/drm/gpu/drm-kms-helpers.html?highlight=drm_get_panel_orientation_quirk#c.drm_get_panel_orientation_quirk"

fbcon will head this and rotate in sw, as should any competent compositor
in userspace (but some might not, it depends).

Also ping Hans de Geode if you have any questions, he's done this.
-Daniel

> 
> Cheers,
> 
> Daniel

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch