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

[Maxime Ripard <maxime.ripard@free-electrons.com>]

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On Mon, Oct 16, 2017 at 04:20:32PM +0800, Chen-Yu Tsai wrote:
> > 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?
> 
> This problem was witnessed on the Cubietruck, which has 2GB of RAM.
> 
> I believe devices with less RAM function normally due to the DRAM
> address wrapping around. CMA seems to always allocate its buffer
> at a very high address, close to the end of DRAM. On a 1GB RAM
> device, the physical address would be something like 0x78000000.
> The DRAM address 0x78000000 would access the same DRAM region as
> 0x38000000 on a system, as the DRAM address would only span 0x0 ~
> 0x3fffffff. The bit 0x40000000 is non-functional in this case.
> 
> However on the Cubietruck, the DRAM is 2GB. The physical address
> is 0x40000000 ~ 0xbfffffff. The buffer would be something like
> 0xb8000000. But the DRAM address span 0x0 ~ 0x7fffffff, meaning
> the buffer address wraps around to 0x38000000, which is wrong.
> The correct DRAM address for it should be 0x78000000.

That looks great. Can you put that in the commit log?

Thanks!
Maxime

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

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