Re: [RFC PATCH] dma: coherent: respect to device 'dma-coherent' property
From: Robin Murphy
Date: Mon Apr 17 2023 - 08:28:07 EST
On 2023-04-17 03:06, Z.Q. Hou wrote:
Hi Christoph,
-----Original Message-----
From: Christoph Hellwig <hch@xxxxxx>
Sent: Sunday, April 16, 2023 2:30 PM
To: Z.Q. Hou <zhiqiang.hou@xxxxxxx>
Cc: iommu@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; hch@xxxxxx;
m.szyprowski@xxxxxxxxxxx; robin.murphy@xxxxxxx
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@xxxxxxx>
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.
Yes, in principle this should apply similarly to restricted DMA or
confidential compute VMs where DMA buffers are to be allocated from a
predetermined shared memory area, and a DT reserved-memory region is
used as a coherent pool to achieve that. Quite likely that so far this
has only been done with non-coherent hardware or in software models
where a mismatch in nominal cacheability wasn't noticeable.
It's a bit niche, but not entirely unreasonable.
Thanks,
Robin.