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

[Chen-Yu Tsai]

On Tue, Oct 17, 2017 at 4:15 AM, Maxime Ripard
<maxime.ripard at free-electrons.com> wrote:
> 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?

Will do.

ChenYu