Re: [PATCH v2 10/10] drm/ofdrm: Support color management

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2022-07-27 at 10:41 +0200, Thomas Zimmermann wrote:
> 
> > > +static void __iomem *ofdrm_mach64_cmap_ioremap(struct ofdrm_device *odev,
> > > +					       struct device_node *of_node,
> > > +					       u64 fb_base)
> > > +{
> > > +	struct drm_device *dev = &odev->dev;
> > > +	u64 address;
> > > +	void __iomem *cmap_base;
> > > +
> > > +	address = fb_base & 0xff000000ul;
> > > +	address += 0x7ff000;
> > > +
> > 
> > It would be good to know where these addresses are coming from. Maybe some
> > constant macros or a comment ? Same for the other places where addresses
> > and offsets are used.
> 
> I have no idea where these values come from. I took them from offb. And 
> I suspect that some of these CMAP helpers could be further merged if 
> only it was clear where the numbers come from.  But as i don't have the 
> equipment for testing, I took most of this literally as-is from offb.

Ancient black magic :-) Old ATI mach64 chips had the registers sitting
at the end of the framebuffer. You can find an equivalent in
drivers/video/aty/atyfb_base.c:atyfb_setup_generic():

	raddr = addr + 0x7ff000UL;

Cheers,
Ben.