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Date:   Thu, 3 Sep 2020 10:42:26 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc:     Christoph Hellwig <hch@lst.de>, alsa-devel@alsa-project.org,
        linux-ia64@vger.kernel.org, linux-doc@vger.kernel.org,
        nouveau@lists.freedesktop.org, linux-nvme@lists.infradead.org,
        linux-kernel@vger.kernel.org,
        "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
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        Joonyoung Shim <jy0922.shim@samsung.com>,
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        Kyungmin Park <kyungmin.park@samsung.com>,
        Ben Skeggs <bskeggs@redhat.com>,
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        Tom Lendacky <thomas.lendacky@amd.com>,
        Pawel Osciak <pawel@osciak.com>,
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        netdev@vger.kernel.org, Seung-Woo Kim <sw0312.kim@samsung.com>,
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Subject: Re: [PATCH 22/28] sgiseeq: convert from dma_cache_sync to
 dma_sync_single_for_device
Message-ID: <20200903084226.GA24410@lst.de>
References: <20200819065555.1802761-1-hch@lst.de> <20200819065555.1802761-23-hch@lst.de> <20200901152209.GA14288@alpha.franken.de> <20200901171241.GA20685@alpha.franken.de> <20200901171627.GA8255@lst.de> <20200901173810.GA25282@alpha.franken.de> <20200902213809.GA7998@alpha.franken.de>
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On Wed, Sep 02, 2020 at 11:38:09PM +0200, Thomas Bogendoerfer wrote:
> the patch below fixes the problem.

But is very wrong unfortunately.

>  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_device(dev->dev.parent, VIRT_TO_DMA(sp, addr),
> +                       sizeof(struct sgiseeq_rx_desc), DMA_FROM_DEVICE);
>  }
>  
>  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_TO_DEVICE);

This is not how the DMA API works.  You can only call
dma_sync_single_for_{device,cpu} with the direction that the memory
was mapped.  It then transfer ownership to the device or the cpu,
and the ownership of the memory is a fundamental concept that allows
for reasoning about the caching interaction.

>  }
>  
> -- 
> Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
> good idea.                                                [ RFC1925, 2.3 ]
---end quoted text---