etnaviv: PHYS_OFFSET usage

[Lucas Stach]

Hi Alexey,

Am Mittwoch, den 15.11.2017, 16:24 +0000 schrieb Alexey Brodkin:
> Hi Lucas,
> 
> As we discussed on ELCE last month in Prague we have Vivante GPU
> built-in our new ARC HSDK development board.
> 
> And even though [thanks to your suggestions] I got Etnaviv driver
> working perfectly fine on our board I faced one quite a tricky
> situation [which I dirty worked-around for now].
> 
> Etnaviv driver uses some PHYS_OFFSET define which is not very
> usual across all architectures and platforms supported by Linux kernel.
> 
> In fact for ARC we don't have PHYS_OFFSET defined [yet].
> And I'm wondering how to get this resolved.
> 
> Essentially we have 2 options:
>  1. Define PHYS_OFFSET for ARC (and later for other arches once needed)
>  2. Replace PHYS_OFFSET with something else in etnaviv sources.
> 
> Even though (1) seems to be the simplest solution is doesn't look very nice
> because it seems to be quite ARM-specific but not something really generic
> and portable.
> 
> As for (2) frankly I din't quite understand why do we really care about
> DDR start offset in the GPU driver. If some more light could be shed on this
> topic probably we'll figure out what would be more elegant solution.

Basically the GPU has a linear address window which is 2GB in size and
all GPU command buffers must be mapped through this window. The window
has a base offset, so we can move it to point to different locations in
the physical address space of the system.

Etnaviv uses the PHYS_OFFSET to find out where in the physical address
space the RAM starts. If the start of RAM is above the 2GB mark we
_must_ use the linear window in order to make the command buffers
available to the GPU.

I'm not aware of any other kernel API that would allow us to find the
start of RAM. If there is I would be happy to replace the PHYS_OFFSET
stuff. If you don't like to copy the PHYS_OFFSET stuff to ARC, you
would need to introduce some new API, which allows us to retrieve this
information.

Regards,
Lucas