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

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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.



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