Re: DMA direct mapping fix for 5.4 and earlier stable branches

Robin Murphy
2021-02-09T12:47:23Z

On 2021-02-09 12:36, Sumit Garg wrote:
> 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].

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.