netdev - Re: [PATCH net-next 2/3] net: page_pool: add the possibility to sync DMA memory for non-coherent devices

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Date:   Thu, 14 Nov 2019 20:53:26 +0200
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, netdev@...r.kernel.org,
        lorenzo.bianconi@...hat.com, davem@...emloft.net,
        thomas.petazzoni@...tlin.com, brouer@...hat.com,
        matteo.croce@...hat.com
Subject: Re: [PATCH net-next 2/3] net: page_pool: add the possibility to sync
 DMA memory for non-coherent devices

[...]
> > index 2cbcdbdec254..defbfd90ab46 100644
> > --- a/include/net/page_pool.h
> > +++ b/include/net/page_pool.h
> > @@ -65,6 +65,9 @@ struct page_pool_params {
> >  	int		nid;  /* Numa node id to allocate from pages from */
> >  	struct device	*dev; /* device, for DMA pre-mapping purposes */
> >  	enum dma_data_direction dma_dir; /* DMA mapping direction */
> > +	unsigned int	max_len; /* max DMA sync memory size */
> > +	unsigned int	offset;  /* DMA addr offset */
> > +	u8 sync;
> >  };
> 
> How about using PP_FLAG_DMA_SYNC instead of another flag word?
> (then it can also be gated on having DMA_MAP enabled)

You mean instead of the u8?
As you pointed out on your V2 comment of the mail, some cards don't sync back to
device.
As the API tries to be generic a u8 was choosen instead of a flag to cover these
use cases. So in time we'll change the semantics of this to 'always sync', 'dont
sync if it's an skb-only queue' etc.
The first case Lorenzo covered is sync the required len only instead of the full
buffer


Thanks
/Ilias