linux-kernel - RE: [RFC PATCH] dma: coherent: respect to device 'dma-coherent' property

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Date:   Mon, 17 Apr 2023 02:06:55 +0000
From:   "Z.Q. Hou" <zhiqiang.hou@....com>
To:     Christoph Hellwig <hch@....de>
CC:     "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "robin.murphy@....com" <robin.murphy@....com>
Subject: RE: [RFC PATCH] dma: coherent: respect to device 'dma-coherent'
 property

Hi Christoph,

> -----Original Message-----
> From: Christoph Hellwig <hch@....de>
> Sent: Sunday, April 16, 2023 2:30 PM
> To: Z.Q. Hou <zhiqiang.hou@....com>
> Cc: iommu@...ts.linux.dev; linux-kernel@...r.kernel.org; hch@....de;
> m.szyprowski@...sung.com; robin.murphy@....com
> Subject: Re: [RFC PATCH] dma: coherent: respect to device 'dma-coherent'
> property
> 
> On Fri, Apr 14, 2023 at 04:03:07PM +0800, Zhiqiang Hou wrote:
> > From: Hou Zhiqiang <Zhiqiang.Hou@....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.
> 
> 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 respectively.
I created a virtual device for Linux and RTOS communication using shared memory.
In Linux side, I created a per-device dma memory pool and added 'dma-coherent'
for the virtual device, but the data in shared memory can't be sync up, finally found
the per-device dma pool is always mapped as uncached, so submitted this fix patch.

Thanks,
Zhiqiang