Re: [RFC 14/16] drm/nouveau/fb: add GK20A support

[Alexandre Courbot]

On Sun, Feb 2, 2014 at 8:58 AM, Lucas Stach <dev@xxxxxxxxxx> wrote:
> Am Samstag, den 01.02.2014, 18:28 -0500 schrieb Ilia Mirkin:
>> On Sat, Feb 1, 2014 at 8:40 AM, Lucas Stach <dev@xxxxxxxxxx> wrote:
>> > Am Samstag, den 01.02.2014, 12:16 +0900 schrieb Alexandre Courbot:
>> >> Add a clumsy-but-working FB support for GK20A. This chip only uses system
>> >> memory, so we allocate a big chunk using CMA and let the existing memory
>> >> managers work on it.
>> >>
>> >> A better future design would be to allocate objects directly from system
>> >> memory without having to suffer from the limitations of a large,
>> >> contiguous pool.
>> >>
>> > I don't know if Tegra124 is similar to 114 in this regard [hint: get the
>> > TRM out :)], but if you go for a dedicated VRAM allocator, wouldn't it
>> > make sense to take a chunk of the MMIO overlaid memory for this when
>> > possible, rather than carving this out of CPU accessible mem?
>>
>> This is probably a stupid question... what do you need VRAM for
>> anyways? In _theory_ it's an abstraction to talk about memory that's
>> not accessible by the CPU. This is obviously not the case here, and
>> presumably the GPU can access all the memory in the system, so it can
>> be all treated as "GART" memory... AFAIK all accesses are behind the
>> in-GPU MMU, so contiguous physical memory isn't an issue either. In
>> practice, I suspect nouveau automatically sticks certain things into
>> vram (gpuobj's), but it should be feasible to make them optionally use
>> GART memory when VRAM is not available. I haven't really looked at the
>> details though, perhaps that's a major undertaking.
>>
>>   -ilia
>>
> If it's similar to the Tegar114 there actually is memory that isn't
> accessible from the CPU. About 2GB of the address space is overlaid with
> MMIO for the devices, so in a 4GB system you potentially have 2GB of RAM
> that's only visible for the devices.
>
> But yes in general nouveau should just fall back to a GART placement if
> VRAM isn't available.

With the limited time I spent studying it, it seems to me that Nouveau
has a strong dependency on VRAM. For gpuobjects indeed (that one could
be workarounded with a new instmem driver I suppose), and also for
TTM: objects placed in TTM_PL_VRAM are handled by the VRAM manager,
which requires a nouveau_ram instance in the FB. Actually the FB also
seems to assume the presence of a dedicated video RAM.

So while I agree that getting rid of VRAM altogether would be the most
logical solution, I have not found a way to do so for the moment.

T124's GPU actually sees the same physical address space as the CPU,
so memory management should be simplified thanks to that (you could
enable the SMMU and make things more interesting/complex, but for now
it seems untimely to even consider doing so). Actually even the
concept of a GART is not needed here: all your memory management needs
could be fulfilled by getting pages with alloc_page() and arranging
them using the GMMU. No GART, no BAR (at least for the purpose of
mapping objects for CPU access), no PRAMIN.

I really wonder how that picture would fit within Nouveau, and it is
quite likely that there is an elegant solution to this problem already
that my lack of understanding of Nouveau prevents me from seeing.
That's why your thoughts on this matter would be greatly appreciated.

Thanks,
Alex.
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