linux-kernel - Re: [PATCH 3/5] Move dma_ops from archdata into struct device

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Date:   Fri, 9 Dec 2016 11:46:30 -0800
From:   Bart Van Assche <bart.vanassche@...disk.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Christoph Hellwig <hch@....de>
CC:     Doug Ledford <dledford@...hat.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>,
        <linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] Move dma_ops from archdata into struct device

On 12/09/2016 11:13 AM, David Woodhouse wrote:
> On Fri, 2016-12-09 at 19:22 +0100, Christoph Hellwig wrote:
>> We'll need a bit of a wieder audience for this I think..
>>
>> On Wed, Dec 07, 2016 at 05:11:28PM -0800, Bart Van Assche wrote:
>>> Additionally, introduce set_dma_ops(). A later patch will introduce a
>>> call to that function in the RDMA drivers that will be modified to use
>>> dma_noop_ops.
>>
>> This looks good to me, and we had a lot of talk about this for other
>> purposes for a while.
>
> Hm, I'm not convinced we want per-device dma_ops. What we want is per-
> device IOMMU ops, and any dma_ops are just a generic or platform-
> specific (in some cases) wrapper around those. We shouldn't normally
> need per-device DMA ops at all.

Hello David,

Can you recommend an approach for e.g. the qib driver 
(drivers/infiniband/hw/qib)? That driver uses the CPU (PIO) instead of 
DMA to transfer data to a PCIe device. Sorry but I don't see how 
per-device IOMMU ops would allow to avoid that e.g. a cache flush is 
triggered before PIO starts.

Bart.