Subject: Re: [PATCH 22/28] sgiseeq: convert from dma_cache_sync to dma_sync_single_for_device

From: Thomas Bogendoerfer <[email-masked]>
Date: Tue, 1 Sep 2020 19:12:41 +0200

On Tue, Sep 01, 2020 at 05:22:09PM +0200, Thomas Bogendoerfer wrote:
> On Wed, Aug 19, 2020 at 08:55:49AM +0200, Christoph Hellwig wrote:
> > Use the proper modern API to transfer cache ownership for incoherent DMA.
> > 
> > Signed-off-by: Christoph Hellwig <[email-masked]>
> > ---
> >  drivers/net/ethernet/seeq/sgiseeq.c | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/seeq/sgiseeq.c b/drivers/net/ethernet/seeq/sgiseeq.c
> > index 39599bbb5d45b6..f91dae16d69a19 100644
> > --- a/drivers/net/ethernet/seeq/sgiseeq.c
> > +++ b/drivers/net/ethernet/seeq/sgiseeq.c
> > @@ -112,14 +112,18 @@ struct sgiseeq_private {
> >  
> >  static inline void dma_sync_desc_cpu(struct net_device *dev, void *addr)
> >  {
> > -	dma_cache_sync(dev->dev.parent, addr, sizeof(struct sgiseeq_rx_desc),
> > -		       DMA_FROM_DEVICE);
> > +	struct sgiseeq_private *sp = netdev_priv(dev);
> > +
> > +	dma_sync_single_for_cpu(dev->dev.parent, VIRT_TO_DMA(sp, addr),
> > +			sizeof(struct sgiseeq_rx_desc), DMA_BIDIRECTIONAL);
> >  }
> >  
> >  static inline void dma_sync_desc_dev(struct net_device *dev, void *addr)
> >  {
> > -	dma_cache_sync(dev->dev.parent, addr, sizeof(struct sgiseeq_rx_desc),
> > -		       DMA_TO_DEVICE);
> > +	struct sgiseeq_private *sp = netdev_priv(dev);
> > +
> > +	dma_sync_single_for_device(dev->dev.parent, VIRT_TO_DMA(sp, addr),
> > +			sizeof(struct sgiseeq_rx_desc), DMA_BIDIRECTIONAL);
> >  }
> 
> this breaks ethernet on IP22 completely, but I haven't figured out why, yet.

the problem is that dma_sync_single_for_cpu() doesn't flush anything
for IP22, because it only flushes for CPUs which do speculation. So
either MIPS arch_sync_dma_for_cpu() should always flush or sgiseeq
needs to use a different sync funktion, when it wants to re-read descriptors
from memory.

Thomas.

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