Re: [PATCH v6 2/2] habanalabs: add support for dma-buf exporter

[Jason Gunthorpe]

On Thu, Sep 30, 2021 at 03:46:35PM +0300, Oded Gabbay wrote:

> After reading the kernel iommu code, I think this is not relevant
> here, and I'll add a comment appropriately but I'll also write it
> here, and please correct me if my understanding is wrong.
> 
> The memory behind this specific dma-buf has *always* resided on the
> device itself, i.e. it lives only in the 'device' domain (after all,
> it maps a PCI bar address which points to the device memory).
> Therefore, it was never in the 'CPU' domain and hence, there is no
> need to perform a sync of the memory to the CPU's cache, as it was
> never inside that cache to begin with.
> 
> This is not the same case as with regular memory which is dma-mapped
> and then copied into the device using a dma engine. In that case,
> the memory started in the 'CPU' domain and moved to the 'device'
> domain. When it is unmapped it will indeed be recycled to be used
> for another purpose and therefore we need to sync the CPU cache.
> 
> Is my understanding correct ?

It makes sense to me

Jason