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

[Sumit Garg]

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].

[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.