On 2021-02-09 12:36, Sumit Garg wrote:
Hi Christoph,
On Tue, 9 Feb 2021 at 15:06, Christoph Hellwig <hch@xxxxxx> 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].
It's not a bug, it's a fundamental design failure. dma_get_sgtable() has
only ever sort-of-worked for DMA buffers that come from CMA or regular
page allocations. In particular, a "no-map" DMA pool is not backed by
kernel memory, so does not have any corresponding page structs, so it's
impossible to generate a *valid* scatterlist to represent memory from
that pool, regardless of what you might get away with provided you don't
poke too hard at it.
It is not a good API...
Robin.
[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.
