netdev - Re: [PATCH v2 net-next] net: socionext: get rid of huge dma sync in netsec_alloc_rx

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Message-ID: <20200110181940.GB31419@localhost.localdomain>
Date:   Fri, 10 Jan 2020 19:19:40 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH v2 net-next] net: socionext: get rid of huge dma sync in
 netsec_alloc_rx_data

> On Fri, 10 Jan 2020 16:34:13 +0100
> Lorenzo Bianconi <lorenzo@...nel.org> wrote:
> 
> > > On Fri, Jan 10, 2020 at 02:57:44PM +0100, Lorenzo Bianconi wrote:  
> > > > Socionext driver can run on dma coherent and non-coherent devices.
> > > > Get rid of huge dma_sync_single_for_device in netsec_alloc_rx_data since
> > > > now the driver can let page_pool API to managed needed DMA sync
> > > > 
> > > > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > > > ---
> > > > Changes since v1:
> > > > - rely on original frame size for dma sync
> > > > ---
> > > >  drivers/net/ethernet/socionext/netsec.c | 43 +++++++++++++++----------
> > > >  1 file changed, 26 insertions(+), 17 deletions(-)
> > > >   
> > 
> > [...]
> > 
> > > > @@ -883,6 +881,8 @@ static u32 netsec_xdp_xmit_back(struct netsec_priv *priv, struct xdp_buff *xdp)
> > > >  static u32 netsec_run_xdp(struct netsec_priv *priv, struct bpf_prog *prog,
> > > >  			  struct xdp_buff *xdp)
> > > >  {
> > > > +	struct netsec_desc_ring *dring = &priv->desc_ring[NETSEC_RING_RX];
> > > > +	unsigned int len = xdp->data_end - xdp->data;  
> > > 
> > > We need to account for XDP expanding the headers as well here. 
> > > So something like max(xdp->data_end(before bpf), xdp->data_end(after bpf)) -
> > > xdp->data (original)  
> > 
> > correct, the corner case that is not covered at the moment is when data_end is
> > moved forward by the bpf program. I will fix it in v3. Thx
> 
> Maybe we can simplify do:
> 
>  void *data_start = NETSEC_RXBUF_HEADROOM + xdp->data_hard_start;
>  unsigned int len = xdp->data_end - data_start;
> 

Hi Jesper,

please correct me if I am wrong but this seems to me the same as v2. The leftover
corner case is if xdp->data_end is moved 'forward' by the bpf program (I guess
it is possible, right?). In this case we will not sync xdp->data_end(new) - xdp->data_end(old)

Regards,
Lorenzo

> The cache-lines that need to be flushed/synced for_device is the area
> used by NIC DMA engine.  We know it will always start at a certain
> point (given driver configured hardware to this).
> 
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
> 

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