DMA direct mapping fix for 5.4 and earlier stable branches

[Sumit Garg <sumit.garg@linaro.org>]

Hi Christoph, Greg,

Currently we are observing an incorrect address translation
corresponding to DMA direct mapping methods on 5.4 stable kernel while
sharing dmabuf from one device to another where both devices have
their own coherent DMA memory pools.

I am able to root cause this issue which is caused by incorrect virt
to phys translation for addresses belonging to vmalloc space using
virt_to_page(). But while looking at the mainline kernel, this patch
[1] changes address translation from virt->to->phys to dma->to->phys
which fixes the issue observed on 5.4 stable kernel as well (minimal
fix [2]).

So I would like to seek your suggestion for backport to stable kernels
(5.4 or earlier) as to whether we should backport the complete
mainline commit [1] or we should just apply the minimal fix [2]?

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=34dc0ea6bc960f1f57b2148f01a3f4da23f87013
[2] minimal fix required for 5.4 stable kernel:

commit bb0b3ff6e54d78370b6b0c04426f0d9192f31795
Author: Sumit Garg <sumit.garg@linaro.org>
Date:   Wed Feb 3 13:08:37 2021 +0530

    dma-mapping: Fix common get_sgtable and mmap methods

    Currently common get_sgtable and mmap methods can only handle normal
    kernel addresses leading to incorrect handling of vmalloc addresses which
    is common means for DMA coherent memory mapping.

    So instead of cpu_addr, directly decode the physical address from
dma_addr and
    hence decode corresponding page and pfn values. In this way we can handle
    normal kernel addresses as well as vmalloc addresses.

    This fix is inspired from following mainline commit:

    34dc0ea6bc96 ("dma-direct: provide mmap and get_sgtable method overrides")

    This fixes an issue observed during dmabuf sharing from one device to
    another where both devices have their own coherent DMA memory pools.

    Signed-off-by: Sumit Garg <sumit.garg@linaro.org>

diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 8682a53..034bbae 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -127,7 +127,7 @@ int dma_common_get_sgtable(struct device *dev,
struct sg_table *sgt,
                        return -ENXIO;
                page = pfn_to_page(pfn);
        } else {
-               page = virt_to_page(cpu_addr);
+               page = pfn_to_page(PHYS_PFN(dma_to_phys(dev, dma_addr)));
        }

        ret = sg_alloc_table(sgt, 1, GFP_KERNEL);
@@ -214,7 +214,7 @@ int dma_common_mmap(struct device *dev, struct
vm_area_struct *vma,
                if (!pfn_valid(pfn))
                        return -ENXIO;
        } else {
-               pfn = page_to_pfn(virt_to_page(cpu_addr));
+               pfn = PHYS_PFN(dma_to_phys(dev, dma_addr));
        }

        return remap_pfn_range(vma, vma->vm_start, pfn + vma->vm_pgoff,