On Thu, Jul 04, 2019 at 07:39:44PM +0200, Jesper Dangaard Brouer wrote:
> On Thu, 4 Jul 2019 17:46:09 +0300
> Ilias Apalodimas <ilias.apalodi...@linaro.org> wrote:
>
> > Quoting Arnd,
> >
> > We have to do a sync_single_for_device /somewhere/ before the
> > buffer is given to the device. On a non-cache-coherent machine with
> > a write-back cache, there may be dirty cache lines that get written back
> > after the device DMA's data into it (e.g. from a previous memset
> > from before the buffer got freed), so you absolutely need to flush any
> > dirty cache lines on it first.
> >
> > Since the coherency is configurable in this device make sure we cover
> > all configurations by explicitly syncing the allocated buffer for the
> > device before refilling it's descriptors
> >
> > Signed-off-by: Ilias Apalodimas <ilias.apalodi...@linaro.org>
> > ---
> >
> > Changes since V1:
> > - Make the code more readable
> >
> > drivers/net/ethernet/socionext/netsec.c | 7 ++++++-
> > 1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/ethernet/socionext/netsec.c
> > b/drivers/net/ethernet/socionext/netsec.c
> > index 5544a722543f..ada7626bf3a2 100644
> > --- a/drivers/net/ethernet/socionext/netsec.c
> > +++ b/drivers/net/ethernet/socionext/netsec.c
> > @@ -727,21 +727,26 @@ static void *netsec_alloc_rx_data(struct netsec_priv
> > *priv,
> > {
> >
> > struct netsec_desc_ring *dring = &priv->desc_ring[NETSEC_RING_RX];
> > + enum dma_data_direction dma_dir;
> > + dma_addr_t dma_start;
> > struct page *page;
> >
> > page = page_pool_dev_alloc_pages(dring->page_pool);
> > if (!page)
> > return NULL;
> >
> > + dma_start = page_pool_get_dma_addr(page);
> > /* We allocate the same buffer length for XDP and non-XDP cases.
> > * page_pool API will map the whole page, skip what's needed for
> > * network payloads and/or XDP
> > */
> > - *dma_handle = page_pool_get_dma_addr(page) + NETSEC_RXBUF_HEADROOM;
> > + *dma_handle = dma_start + NETSEC_RXBUF_HEADROOM;
> > /* Make sure the incoming payload fits in the page for XDP and non-XDP
> > * cases and reserve enough space for headroom + skb_shared_info
> > */
> > *desc_len = PAGE_SIZE - NETSEC_RX_BUF_NON_DATA;
> > + dma_dir = page_pool_get_dma_dir(dring->page_pool);
> > + dma_sync_single_for_device(priv->dev, dma_start, PAGE_SIZE, dma_dir);
>
> It's it costly to sync_for_device the entire page size?
>
> E.g. we already know that the head-room is not touched by device. And
> we actually want this head-room cache-hot for e.g. xdp_frame, thus it
> would be unfortunate if the head-room is explicitly evicted from the
> cache here.
>
> Even smarter, the driver could do the sync for_device, when it
> release/recycle page, as it likely know the exact length that was used
> by the packet.
It does sync for device when recycling takes place in XDP_TX with the correct
size.
I guess i can explicitly sync on the xdp_return_buff cases, and
netsec_setup_rx_dring() instead of the generic buffer allocation
I'll send a V3
Thanks!
/Ilias
