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SubjectRe: DMA direct mapping fix for 5.4 and earlier stable branches
Hi Christoph,

On Tue, 9 Feb 2021 at 15:06, Christoph Hellwig <hch@lst.de> wrote:
>
> On Tue, Feb 09, 2021 at 10:23:12AM +0100, Greg KH wrote:
> > > From the view point of ZeroCopy using DMABUF, is 5.4 not
> > > mature enough, and is 5.10 enough mature ?
> > > This is the most important point for judging migration.
> >
> > How do you judge "mature"?
> >
> > And again, if a feature isn't present in a specific kernel version, why
> > would you think that it would be a viable solution for you to use?
>
> I'm pretty sure dma_get_sgtable has been around much longer and was
> supposed to work, but only really did work properly for arm32, and
> for platforms with coherent DMA. I bet he is using non-coherent arm64,

It's an arm64 platform using coherent DMA where device coherent DMA
memory pool is defined in the DT as follows:

reserved-memory {
#address-cells = <2>;
#size-cells = <2>;
ranges;

<snip>
encbuffer: encbuffer@0xb0000000 {
compatible = "shared-dma-pool";
reg = <0 0xb0000000 0 0x08000000>; // this
area used with dma-coherent
no-map;
};
<snip>
};

Device is dma-coherent as per following DT property:

codec {
compatible = "socionext,uniphier-pxs3-codec";
<snip>
memory-region = <&encbuffer>;
dma-coherent;
<snip>
};

And call chain to create device coherent DMA pool is as follows:

rmem_dma_device_init();
dma_init_coherent_memory();
memremap();
ioremap_wc();

which simply maps coherent DMA memory into vmalloc space on arm64.

The thing I am unclear is why this is called a new feature rather than
a bug in dma_common_get_sgtable() which is failing to handle vmalloc
addresses? While at the same time DMA debug APIs specifically handle
vmalloc addresses [1].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/dma/debug.c?h=linux-5.4.y#n1462

-Sumit

> and it would be broken for other drivers there as well if people did
> test them, which they apparently so far did not.

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