Re: [PATCH net-next v2 3/5] page_pool: Set `dma_sync` to false for devmem memory provider

[Jakub Kicinski <kuba@kernel.org>]

On Thu,  7 Nov 2024 21:23:07 +0000 Mina Almasry wrote:
> From: Samiullah Khawaja <skhawaja@google.com>
> 
> Move the `dma_map` and `dma_sync` checks to `page_pool_init` to make
> them generic. Set dma_sync to false for devmem memory provider because
> the dma_sync APIs should not be used for dma_buf backed devmem memory
> provider.

Let's start from specifying the expectations from the drivers,
and documenting them under Documentation/

We should require that drivers set PP_FLAG_DMA_SYNC_DEV on the page
pool and always use (a new form of) page_pool_dma_sync_for_cpu()
to sync for CPU.

Behind the scenes we will configure the PP to act accordingly.
For dmabuf backed PP I suspect we just say "syncing is
the responsibility of userspace", and configure the PP
to do no syncing at all.

For other providers we can keep the syncing enabled. But drivers
shouldn't be allowed to use any netmem if they don't set
PP_FLAG_DMA_SYNC_DEV, just to keep things uniform.