Re: [linux-sunxi] Re: [PATCH 5/7] drm/sun4i: backend: Offset layer buffer address by DRAM starting address

[icenowy]

å 2017-10-16 16:00ïMaxime Ripard åéï
> Hi,
>
> I've applied all the other patches.
>
> On Sat, Oct 14, 2017 at 12:02:50PM +0800, Chen-Yu Tsai wrote:
> > The display backend, as well as other peripherals that have a DRAM
> > clock gate and access DRAM directly, bypassing the system bus,
> > address the DRAM starting from 0x0, while physical addresses the
> > system uses starts from 0x40000000 (or 0x20000000 in A80's case).
> >
> > Correct the address configured into the backend layer registers
> > by PHYS_OFFSET to account for this.
>
> However, I'm a bit confused at this.
>
> The driver has been working so far, which it wouldn't have been able
> to if the address was wrong. How was this problem noticed, and how can
> that fix not be an issue in itself?

On devices with <=1GiB (0x40000000) DRAM, the DRAM access will just wrap
back and no problem would be seen.

> Thanks!
> Maxime
>
> --
> Maxime Ripard, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com