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

From: Christoph Hellwig <[email-masked]>
Date: Thu, 3 Sep 2020 10:46:43 +0200

On Thu, Sep 03, 2020 at 10:43:02AM +0200, Christoph Hellwig wrote:
> On Tue, Sep 01, 2020 at 07:38:10PM +0200, Thomas Bogendoerfer wrote:
> > this is the problem:
> > 
> >        /* Always check for received packets. */
> >         sgiseeq_rx(dev, sp, hregs, sregs);
> > 
> > so the driver will look at the rx descriptor on every interrupt, so
> > we cache the rx descriptor on the first interrupt and if there was
> > $no rx packet, we will only see it, if cache line gets flushed for
> > some other reason.
> 
> That means a transfer back to device ownership is missing after a
> (negative) check.

E.g. something like this for the particular problem, although there
might be other hiding elsewhere:

diff --git a/drivers/net/ethernet/seeq/sgiseeq.c b/drivers/net/ethernet/seeq/sgiseeq.c
index 8507ff2420143a..a1c7be8a0d1e5d 100644
--- a/drivers/net/ethernet/seeq/sgiseeq.c
+++ b/drivers/net/ethernet/seeq/sgiseeq.c
@@ -403,6 +403,8 @@ static inline void sgiseeq_rx(struct net_device *dev, struct sgiseeq_private *sp
 		rd = &sp->rx_desc[sp->rx_new];
 		dma_sync_desc_cpu(dev, rd);
 	}
+	dma_sync_desc_dev(dev, rd);
+
 	dma_sync_desc_cpu(dev, &sp->rx_desc[orig_end]);
 	sp->rx_desc[orig_end].rdma.cntinfo &= ~(HPCDMA_EOR);
 	dma_sync_desc_dev(dev, &sp->rx_desc[orig_end]);