Subject: Re: DPAA2 triggers, [PATCH] dma debug: report -EEXIST errors in add_dma_entry

From: Christoph Hellwig <[email-masked]>
Date: Mon, 11 Oct 2021 13:47:06 +0200

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?