linux-kernel - Re: [PATCH 0/5 v3] Fix NVMe driver support on Power with 32-bit DMA

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Date:	Tue, 27 Oct 2015 16:40:19 -0700
From:	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>
To:	Julian Calaby <julian.calaby@...il.com>
Cc:	David Miller <davem@...emloft.net>, willy@...ux.intel.com,
	keith.busch@...el.com, benh@...nel.crashing.org, paulus@...ba.org,
	mpe@...erman.id.au, aik@...abs.ru, david@...son.dropbear.id.au,
	Christoph Hellwig <hch@...radead.org>,
	linux-nvme@...ts.infradead.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linuxppc-dev@...ts.ozlabs.org,
	sparclinux <sparclinux@...r.kernel.org>
Subject: Re: [PATCH 0/5 v3] Fix NVMe driver support on Power with 32-bit DMA

On 28.10.2015 [09:57:48 +1100], Julian Calaby wrote:
> Hi Nishanth,
> 
> On Wed, Oct 28, 2015 at 9:20 AM, Nishanth Aravamudan
> <nacc@...ux.vnet.ibm.com> wrote:
> > On 26.10.2015 [18:27:46 -0700], David Miller wrote:
> >> From: Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>
> >> Date: Fri, 23 Oct 2015 13:54:20 -0700
> >>
> >> > 1) add a generic dma_get_page_shift implementation that just returns
> >> > PAGE_SHIFT
> >>
> >> I won't object to this patch series, but if I had implemented this I
> >> would have required the architectures to implement this explicitly,
> >> one-by-one.  I think it is less error prone and more likely to end
> >> up with all the architectures setting this correctly.
> >
> > Well, looks like I should spin up a v4 anyways for the powerpc changes.
> > So, to make sure I understand your point, should I make the generic
> > dma_get_page_shift a compile-error kind of thing? It will only fail on
> > architectures that actually build the NVME driver (as the only caller).
> > But I'm not sure how exactly to achieve that, if you could give a bit
> > more detail I'd appreciate it!
> 
> He's suggesting that you _don't_ put a generic implementation in
> /include/linux/dma-mapping.h and instead add it to _every_
> architecture.

Ah, I see! Well, I don't know much about the DMA internals of most
architectures -- and my approach kept things functionally the same
everywhere (using PAGE_SHIFT) except:

a) Power, where I know it doesn't work as-is
and
b) sparc, where the code implied that a different value than PAGE_SHIFT
should be used.

Thanks,
Nish

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