Re: [PATCH v4] of: reserved_mem: Restructure call site for dma_contiguous_early_fixup()

[Ye Li]

On 8/11/2025 7:07 PM, Marek Szyprowski wrote:
> On 06.08.2025 19:24, Oreoluwa Babatunde wrote:
> > Restructure the call site for dma_contiguous_early_fixup() to
> > where the reserved_mem nodes are being parsed from the DT so that
> > dma_mmu_remap[] is populated before dma_contiguous_remap() is called.
> >
> > Fixes: 8a6e02d0c00e ("of: reserved_mem: Restructure how the reserved memory regions are processed")
> > Signed-off-by: Oreoluwa Babatunde <oreoluwa.babatunde@xxxxxxxxxxxxxxxx>
> > Tested-by: William Zhang <william.zhang@xxxxxxxxxxxx>
>
> Thanks, applied to dma-mapping-fixes branch.
>
> Best regards

Hi Oreoluwa,

We observed this patch causing kernel boot hang on iMX6 (armv7) 
platforms if using "cma=" kernel parameter. It only happens when the 
size assigned in
"cma=" parameter is smaller than cma default size in dts.

For example, we use "cma=96M" in command line and below reserved memory 
node (160M) in dts.

        reserved-memory {
                #address-cells = <1>;
                #size-cells = <1>;
                ranges;

                linux,cma {
                        compatible = "shared-dma-pool";
                        reusable;
                        size = <0xa000000>;
                        linux,cma-default;
                };
        };

The root cause is this patch moving the dma_contiguous_early_fixup from 
rmem_cma_setup to __reserved_mem_alloc_size. rmem_cma_setup can skip the 
cma reserved memory if command line has cma parameter. However, the 
__reserved_mem_alloc_size won't do it. So this leads to have two cma 
regions added to dma_mmu_remap, one from dts, the other from command 
line. But the reserved memory of memblock that only records the cma from 
command line is inconsistent with dma_mmu_remap.
The dma_contiguous_remap clears the MMU paging for the region of 
dma_mmu_remap firstly, then create a new mapping by iotable_init. For 
the cma from dts, this causes incorrect memory mapping cleared. Then any 
allocation from memblock in iotable_init hitting to the area will meet 
MMU mapping issue.

From commit, I don't understand what issue does this patch fix. Can you 
look into the regression and provide a fix patch.

Best regards,
Ye Li