Re: DMA-buf and uncached system memory

[Thomas Zimmermann]

Hi

Am 15.02.21 um 09:58 schrieb Christian König:
> Hi guys,
>
> we are currently working an Freesync and direct scan out from system 
> memory on AMD APUs in A+A laptops.
>
> On problem we stumbled over is that our display hardware needs to scan 
> out from uncached system memory and we currently don't have a way to 
> communicate that through DMA-buf.
>
> For our specific use case at hand we are going to implement something 
> driver specific, but the question is should we have something more 
> generic for this?

For vmap operations, we return the address as struct dma_buf_map, which 
contains additional information about the memory buffer. In vram 
helpers, we have the interface drm_gem_vram_offset() that returns the 
offset of the GPU device memory.

Would it be feasible to combine both concepts into a dma-buf interface 
that returns the device-memory offset plus the additional caching flag?

There'd be a structure and a getter function returning the structure.

struct dma_buf_offset {
	bool cached;
	u64 address;
};

// return offset in *off
int dma_buf_offset(struct dma_buf *buf, struct dma_buf_off *off);

Whatever settings are returned by dma_buf_offset() are valid while the 
dma_buf is pinned.

Best regards
Thomas

> After all the system memory access pattern is a PCIe extension and as 
> such something generic.
>
> Regards,
> Christian.
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--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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