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

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Date:   Sat, 16 Nov 2019 13:36:30 +0200
From:   Lorenzo Bianconi <lorenzo@...nel.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        ilias.apalodimas@...aro.org, lorenzo.bianconi@...hat.com,
        mcroce@...hat.com
Subject: Re: [PATCH v3 net-next 2/3] net: page_pool: add the possibility to
 sync DMA memory for device

> On Fri, 15 Nov 2019 21:01:38 +0200
> Lorenzo Bianconi <lorenzo@...nel.org> wrote:
> 
> >  static bool __page_pool_recycle_into_ring(struct page_pool *pool,
> > -				   struct page *page)
> > +					  struct page *page,
> > +					  unsigned int dma_sync_size)
> >  {
> >  	int ret;
> >  	/* BH protection not needed if current is serving softirq */
> > @@ -264,6 +285,9 @@ static bool __page_pool_recycle_into_ring(struct page_pool *pool,
> >  	else
> >  		ret = ptr_ring_produce_bh(&pool->ring, page);
> >  
> > +	if (ret == 0 && (pool->p.flags & PP_FLAG_DMA_SYNC_DEV))
> > +		page_pool_dma_sync_for_device(pool, page, dma_sync_size);
> > +
> >  	return (ret == 0) ? true : false;
> >  }
> 
> 
> I do wonder if we should DMA-sync-for-device BEFORE putting page into
> ptr_ring, as this is a channel between several concurrent CPUs.

Hi Jesper,

in this way we can end up syncing the DMA page even if it is unmapped in
__page_pool_clean_page (e.g. if the ptr_ring is full), right?

> 
> 
> 
> > @@ -273,18 +297,22 @@ static bool __page_pool_recycle_into_ring(struct page_pool *pool,
> >   * Caller must provide appropriate safe context.
> >   */
> >  static bool __page_pool_recycle_direct(struct page *page,
> > -				       struct page_pool *pool)
> > +				       struct page_pool *pool,
> > +				       unsigned int dma_sync_size)
> >  {
> >  	if (unlikely(pool->alloc.count == PP_ALLOC_CACHE_SIZE))
> >  		return false;
> >  
> >  	/* Caller MUST have verified/know (page_ref_count(page) == 1) */
> >  	pool->alloc.cache[pool->alloc.count++] = page;
> > +
> > +	if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> > +		page_pool_dma_sync_for_device(pool, page, dma_sync_size);
> >  	return true;
> >  }
> 
> We know __page_pool_recycle_direct() is concurrency safe, and only a
> single (NAPI processing) CPU can enter. (So, the DMA-sync order is not
> wrong here, but it could be swapped)

do you mean move it before putting the page in the cache?

pool->alloc.cache[pool->alloc.count++] = page;

Regards,
Lorenzo

> 
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
> 

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