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

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Message-ID: <306596c4-cc91-4500-b705-50e619e9f38e@oss.qualcomm.com>
Date: Sun, 30 Nov 2025 22:31:16 -0800
From: Oreoluwa Babatunde <oreoluwa.babatunde@....qualcomm.com>
To: Marek Szyprowski <m.szyprowski@...sung.com>, Ye Li <ye.li@....nxp.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 11/28/2025 4:43 AM, Marek Szyprowski wrote:
> On 26.11.2025 02:37, Ye Li wrote:
>> 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.
>>
Hi Ye Li,

Thanks for pointing this out. From what I see in the code, if "cma="
kernel parameter is being used to configure the default cma region, then we
should skip adding the DT defined region to dma_mmu_remap array.

I will work on a fix which does this and share here when it is done.

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

Please see below conversation for details on the original issue this patch fixes:
https://lore.kernel.org/all/5aa94f41-c689-443b-8665-c6913ff5ba8f@broadcom.com/

Thanks,
Oreoluwa