Re: [PATCH v2 13/15] drm/ttm: Add BO and offset arguments for vm_access and vm_fault ttm handlers.

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On 5/18/21 7:17 PM, Christian König wrote:
Am 18.05.21 um 19:10 schrieb Thomas Hellström:

On 5/18/21 5:29 PM, Christian König wrote:


Am 18.05.21 um 17:25 schrieb Thomas Hellström:

On 5/18/21 5:17 PM, Christian König wrote:


Am 18.05.21 um 17:11 schrieb Thomas Hellström:

On 5/18/21 5:07 PM, Christian König wrote:
Am 18.05.21 um 16:55 schrieb Thomas Hellström:
From: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>

This allows other drivers that may not setup the vma in the same way
to use the ttm bo helpers.

Uff can you please explain why exactly you need that?

Providing the BO is not much of a problem, but having the BO at different VMA offsets is really a no-go with TTM.

Christian.

The current i915 uapi is using different offsets for different caching :/. We're currently working around that by using ttm_bo_type_kernel (no TTM vma offset at all) and i915's offset.

Can you instead adjust the offset in the mmap callback like we do for dma-buf?
Will have to take a look.

That's really a no-go what you describe here because it will mess up reverse mapping lockup for buffer movement.

You mean the unmap_mapping_range() stuff? That's not a problem since it's a NOP for kernel ttm buffers, and the i915 move() / swap_notify() takes care of killing the ptes.

That design is a certain NAK from my side for upstreaming this.

PTE handling is the domain of TTM, drivers should never mess with that directly.

Hmm. May I humbly suggest a different view on this:

I agree fully for ttm_bo_type_device bos but for ttm_bo_type_kernel, TTM has no business whatsoever with user-space PTEs. That's really why that bo type exists in the first place. But otoh one can of course argue that then i915 has no business calling the TTM fault helper for these bos.

Well the real question is why does i915 wants to expose kernel BOs to userspace? As the name says only the kernel should be able to access them.

I'd say "kernel" of course refers to how TTM handles them.



So for discrete we can probably do the right thing with ttm_bo_type_device. What worries me a bit is when we get to older hardware support because whatever we do is by definition going to be ugly. At best we might be able to split the address space between i915's mmos, and hand the rest to TTM, modifying offsets as you suggest. That way a TTM call to unmap_mapping_range() would do the right thing, I think.

Well we do all kind of nasty stuff with the offset in DMA-buf, KFD, overlayfs etc. So adjusting the offset inside the kernel is already well supported and used.

What I don't fully understand is your use case here? Can you briefly describe that?

Do you use different bits of the offset to signal what caching behavior you have? And then based on this adjust the pgprot_t in the vma?

Thanks,
Christian.

TBH I'm not completely sure about the history of this. I think the basic idea is that you want to support different vmas with different caching modes, so you never change the vma pgprot after mmap time, like TTM does. Needless to say this only works with Intel processors on integrated and the more I write about this the more I'm convinced that we should perhaps not concern TTM with that at all.

/Thomas




/Thomas


Christian.


While we're in the process of killing that offset flexibility for discrete, we can't do so for older hardware unfortunately.

/Thomas



Christian.





/Thomas







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