Re: dma_mmap_fault discussion

[Thomas HellstrÃm (VMware)]

On 9/6/19 8:32 AM, Christoph Hellwig wrote:
> On Thu, Sep 05, 2019 at 07:05:43PM +0200, Thomas HellstrÃm (VMware) wrote:
> > I took a quick look at the interfaces and have two questions:
> >
> > 1) dma_mmap_prepare(), would it be possible to drop the references to the
> > actual coherent region? The thing is that TTM doesn't know at mmap time what
> > memory will be backing the region. It can be VRAM, coherent memory or system
> I guess we can shift the argument checking into the fault handler.
>
> > 2) @cpu_addr and @dma_addr are the values pointing at the beginning of an
> > allocated chunk, right?
> Yes.
>
> > The reason I'm asking is that TTM's coherent memory
> > pool is sub-allocating from larger chunks into smaller PAGE_SIZE chunks,
> > which means that a TTM buffer object may be randomly split across larger
> > chunks, which means we have to store these values for each PAGE_SiZE chunk.
> For implementations that remap non-contigous regions using vmap we need the
> start cpu address passed, as that is used to find the vm_struct structure
> for it.  That being said I don't see a problem with you keeping track
> of the original start and offset into in your suballocation helpers.

It's definitely possible. I was just wondering whether it was necessary, 
but it seems like it.

Thanks,

Thomas