linux-kernel - Re: [PATCH kernel 0/2] powerpc/dma: Fallback to dma

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Date:   Tue, 27 Oct 2020 09:36:47 +0100
From:   Christoph Hellwig <hch@....de>
To:     Alexey Kardashevskiy <aik@...abs.ru>
Cc:     linuxppc-dev@...ts.ozlabs.org, Christoph Hellwig <hch@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH kernel 0/2] powerpc/dma: Fallback to dma_ops when
 persistent memory present

On Wed, Oct 21, 2020 at 02:20:24PM +1100, Alexey Kardashevskiy wrote:
> This allows mixing direct DMA (to/from RAM) and
> IOMMU (to/from apersistent memory) on the PPC64/pseries
> platform. This was supposed to be a single patch but
> unexpected move of direct DMA functions happened.
> 
> This is based on sha1
> 7cf726a59435 Linus Torvalds "Merge tag 'linux-kselftest-kunit-5.10-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest".
> 
> Please comment. Thanks.

I really don't like your revert.  I'm almost ready to kill of
dma-direct.h, and I really want it private in kernel/dma/, as people
keep adding abuses to drivers.

We have two options here:

 (1) duplicate the code in arch/powerpc/
 (2) add a hook to kernel/dma/

I've not been a fan of (2) in the past, but now that the code is out
of line, and we could make it dependent on a config option only set by
powerpc, I see it as the lesser evil now.