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Date: Fri, 11 Jan 2019 10:27:28 -0800
From: Christoph Hellwig <hch@infradead.org>
To: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Subject: Re: [PATCH] ibmvscsi: use GFP_ATOMIC with dma_alloc_coherent in
 map_sg_data
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On Thu, Jan 10, 2019 at 03:15:35PM -0800, Tyrel Datwyler wrote:
> On a secondary note I was unaware of the GFP_ATOMIC limitations. Should this be
> added to the documentation somewhere? I don't see any mention here form
> DMA-API-HOWTO.txt.

The DMA documentation unfortauntely doesn't seem very good.  It's been
on my todo list to eventually update it, but I'm still discoverying
various warts.

GFP_ATOMIC allocations generally work fine on DMA coherent
architectures, but tend to cause problems on a lot of non-coherent
ones with the notable exceptions of arm and arm64 that go to great
length to introduce special pools for them.  But that code is rarely
exercised, so I found various bugs e.g. in the arm64 iommu code for
this case.

But more importantly there really should be no need for the coherent
allocation from irq context - we only need coherent for descriptors
that don't have clear ownership, and aything allocated in the I/O path
generally has that.