Re: [PATCH v3 net-next 2/3] net: page_pool: add the possibility to sync DMA memory for device

["Jonathan Lemon" <jonathan.lemon@gmail.com>]

On 15 Nov 2019, at 11:01, Lorenzo Bianconi wrote:

> Introduce the following parameters in order to add the possibility to sync
> DMA memory for device before putting allocated pages in the page_pool
> caches:
> - PP_FLAG_DMA_SYNC_DEV: if set in page_pool_params flags, all pages that
>   the driver gets from page_pool will be DMA-synced-for-device according
>   to the length provided by the device driver. Please note DMA-sync-for-CPU
>   is still device driver responsibility
> - offset: DMA address offset where the DMA engine starts copying rx data
> - max_len: maximum DMA memory size page_pool is allowed to flush. This
>   is currently used in __page_pool_alloc_pages_slow routine when pages
>   are allocated from page allocator
> These parameters are supposed to be set by device drivers.
>
> This optimization reduces the length of the DMA-sync-for-device.
> The optimization is valid because pages are initially
> DMA-synced-for-device as defined via max_len. At RX time, the driver
> will perform a DMA-sync-for-CPU on the memory for the packet length.
> What is important is the memory occupied by packet payload, because
> this is the area CPU is allowed to read and modify. As we don't track
> cache-lines written into by the CPU, simply use the packet payload length
> as dma_sync_size at page_pool recycle time. This also take into account
> any tail-extend.
>
> Tested-by: Matteo Croce <mcroce@redhat.com>
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>

Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>