linux-kernel - RE: [RFC PATCH] dma-mapping: create iommu mapping for newly allocated dma coherent mem

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Date:   Tue, 23 Apr 2019 22:09:54 +0000
From:   Laurentiu Tudor <laurentiu.tudor@....com>
To:     Christoph Hellwig <hch@....de>
CC:     "robin.murphy@....com" <robin.murphy@....com>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Leo Li <leoyang.li@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH] dma-mapping: create iommu mapping for newly allocated
 dma coherent mem

Hello,

> -----Original Message-----
> From: Christoph Hellwig <hch@....de>
> Sent: Monday, April 22, 2019 9:11 PM
> 
> On Mon, Apr 22, 2019 at 07:51:25PM +0300, laurentiu.tudor@....com wrote:
> > From: Laurentiu Tudor <laurentiu.tudor@....com>
> >
> > If possible / available call into the DMA API to get a proper iommu
> > mapping and a dma address for the newly allocated coherent dma memory.
> 
> I don't think this is so simple.  The original use case of
> dma_declare_coherent_memory was memory that is local to a device, where
> we copy in data through a MMIO mapping and the device can then access
> it.  This use case still seems to be alive in the ohci-sm501 and
> ohci-tmio drivers.  Going through the iommu in those cases would be
> counter productive.

I had a feeling that I didn't get the whole story and something isn't quite right with this patch. 😊 But I'm happy that we have a discussion started on the topic and, I must say, I'm very interested in getting to the bottom of it (I have some patches enabling SMMU on a couple of NXP chips and depend on resolving this).
I'll try to understand what you're planning but in the meantime please let me know if you think I can be of any help.

---
Thanks & Best Regards, Laurentiu