linux-kernel - Re: [PATCH] dma: DMA_ATTR_SKIP_CPU

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Message-ID: <20230720060742.GA2987@lst.de>
Date:   Thu, 20 Jul 2023 08:07:42 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, linux-doc@...r.kernel.org,
        iommu@...ts.linux.dev
Subject: Re: [PATCH] dma: DMA_ATTR_SKIP_CPU_SYNC documentation tweaks

On Wed, Jul 19, 2023 at 06:15:59AM -0400, Michael S. Tsirkin wrote:
> A recent patchset highlighted to me that DMA_ATTR_SKIP_CPU_SYNC
> might be easily misunderstood.

.. just curious: what patchset is that?  DMA_ATTR_SKIP_CPU_SYNC is
often a bad idea and all users probably could use a really good
audit..

>  #define DMA_ATTR_NO_KERNEL_MAPPING	(1UL << 4)
>  /*
> - * DMA_ATTR_SKIP_CPU_SYNC: Allows platform code to skip synchronization of
> - * the CPU cache for the given buffer assuming that it has been already
> - * transferred to 'device' domain.
> + * DMA_ATTR_SKIP_CPU_SYNC: Allows platform code to skip synchronization of the
> + * CPU and device domains for the given buffer.

While we're at it, I think "allows" is the wrong word here, we really
must skip the synchronization or else we're in trouble.