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From:   "Z.Q. Hou" <zhiqiang.hou@nxp.com>
To:     Christoph Hellwig <hch@lst.de>
CC:     "iommu@lists.linux.dev" <iommu@lists.linux.dev>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "m.szyprowski@samsung.com" <m.szyprowski@samsung.com>,
        "robin.murphy@arm.com" <robin.murphy@arm.com>
Subject: RE: [RFC PATCH] dma: coherent: respect to device 'dma-coherent'
 property
Thread-Topic: [RFC PATCH] dma: coherent: respect to device 'dma-coherent'
 property
Thread-Index: AQHZbqX64aqH8/sqmEm+RldR8MQMVa8tfJIAgAFAtvA=
Date:   Mon, 17 Apr 2023 02:06:55 +0000
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References: <20230414080307.35114-1-Zhiqiang.Hou@nxp.com>
 <20230416063028.GA6276@lst.de>
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Hi Christoph,

> -----Original Message-----
> From: Christoph Hellwig <hch@lst.de>
> Sent: Sunday, April 16, 2023 2:30 PM
> To: Z.Q. Hou <zhiqiang.hou@nxp.com>
> Cc: iommu@lists.linux.dev; linux-kernel@vger.kernel.org; hch@lst.de;
> m.szyprowski@samsung.com; robin.murphy@arm.com
> Subject: Re: [RFC PATCH] dma: coherent: respect to device 'dma-coherent'
> property
>=20
> On Fri, Apr 14, 2023 at 04:03:07PM +0800, Zhiqiang Hou wrote:
> > From: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
> >
> > Currently, the coherent DMA memory is always mapped as writecombine
> > and uncached, ignored the 'dma-coherent' property in device node, this
> > patch is to map the memory as writeback and cached when the device has
> > 'dma-coherent' property.
>=20
> What is the use case here? The somewhat misnamed per-device coherent
> memory is intended for small per-device pools of sram or such used for
> staging memory.

In my case, there are multiple Cortex-A cores within the cluster, in which =
it is
cache coherent, they are split into 2 island for running Linux and RTOS res=
pectively.
I created a virtual device for Linux and RTOS communication using shared me=
mory.
In Linux side, I created a per-device dma memory pool and added 'dma-cohere=
nt'
for the virtual device, but the data in shared memory can't be sync up, fin=
ally found
the per-device dma pool is always mapped as uncached, so submitted this fix=
 patch.

Thanks,
Zhiqiang