linux-kernel - Re: [PATCH v4] of: reserved_mem: Restructure call site for dma_contiguous_early

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Message-ID: <cef8f9eb-88aa-4771-b25b-2cfd1ac2c387@oss.nxp.com>
Date: Wed, 26 Nov 2025 09:37:19 +0800
From: Ye Li <ye.li@....nxp.com>
To: Marek Szyprowski <m.szyprowski@...sung.com>,
 Oreoluwa Babatunde <oreoluwa.babatunde@....qualcomm.com>, robh@...nel.org,
 robin.murphy@....com
Cc: saravanak@...gle.com, quic_obabatun@...cinc.com,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 iommu@...ts.linux.dev, william.zhang@...adcom.com, kernel@....qualcomm.com,
 will@...nel.org, djakov@...nel.org, aisheng.dong@....com, joy.zou@....com,
 frank.li@....com, jason.hui.liu@....com
Subject: Re: [PATCH v4] of: reserved_mem: Restructure call site for
 dma_contiguous_early_fixup()

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@....qualcomm.com>
>> Tested-by: William Zhang <william.zhang@...adcom.com>
> 
> 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