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

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Message-ID: <ae7e7a10-5aa4-4381-a878-101af19714b3@oss.qualcomm.com>
Date: Mon, 1 Dec 2025 22:54:51 -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 11/30/2025 11:51 PM, Marek Szyprowski wrote:
> On 01.12.2025 07:31, Oreoluwa Babatunde wrote:
>> 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.
> 
> I wonder how to avoid adding more such checks to 
> drivers/of/of_reserved_mem.c and making this code even more tangled and 
> spaghetti-like... I've briefly scanned that code and it is already quite 
> hard to follow, especially after commits 8a6e02d0c00e ("of: 
> reserved_mem: Restructure how the reserved memory regions are 
> processed") and 2c223f7239f3 ("of: reserved_mem: Restructure call site 
> for dma_contiguous_early_fixup()")... I wonder how many reserved memory 
> regions are used on real machines? Maybe instead of complicating this 
> code even more it is enough to make this configurable via Kconfig and 
> restore pre-8a6e02d0c00e version?

Hi Marek,

There was a change which attempted a simpler approach of increasing the
size of the static array:
https://lore.kernel.org/all/1650488954-26662-1-git-send-email-quic_pdaly@quicinc.com/

The comment from Rob at the time was to revive another thread which attempted
to dynamically allocate the reserved_mem array like we are doing now.
Dynamic allocation gives more flexibility because we only use the exact
amount of memory that is needed. This can save some memory if that ends
up being smaller than what is specified in MAX_RESERVED_REGIONS.

I do agree that adding another check in of_reserved_mem.c might not be the best
in terms of code complexity, so I'm exploring other options on how to keep things
simpler.

Regards,
Oreoluwa