netdev - Re: DPAA2 triggers, [PATCH] dma debug: report -EEXIST errors in add_dma

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Date:   Mon, 11 Oct 2021 13:47:06 +0200
From:   Christoph Hellwig <hch@....de>
To:     Karsten Graul <kgraul@...ux.ibm.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Hamza Mahfooz <someguy@...ective-light.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Jeremy Linton <jeremy.linton@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: DPAA2 triggers, [PATCH] dma debug: report -EEXIST errors in
 add_dma_entry

On Thu, Oct 07, 2021 at 12:59:32PM +0200, Karsten Graul wrote:
> In our case its really that a buffer is mapped twice for 2 different devices which we use in SMC to provide failover capabilities. We see that -EEXIST is returned when a buffer is mapped for the second device. Since there is a maximum of 2 parallel mappings we never see the warning shown by active_cacheline_inc_overlap() because we don't exceed ACTIVE_CACHELINE_MAX_OVERLAP.

Mapping something twice is possible, but needs special care.
Basically one device always needs to do the first mapping and the other
one needs to use DMA_ATTR_SKIP_CPU_SYNC to opt out of the coherency
protocol.  So we have two TODO items here: 1) the driver needs to use the
above scheme and 2) this dma-debug check needs to understand
DMA_ATTR_SKIP_CPU_SYNC.  Can I trick you into doing both?