Re: [PATCH v2] firmware: qcom_scm: Add a padded page to ensure DMA memory from lower 4GB

[Prakash Gupta]

On 6/24/2024 8:34 PM, Mukesh Ojha wrote:
> On Sun, Jun 23, 2024 at 07:25:23PM -0500, Bjorn Andersson wrote:
>> On Wed, May 29, 2024 at 05:24:29PM GMT, Mukesh Ojha wrote:
>>>
>>>
>>> On 5/27/2024 2:16 AM, Bjorn Andersson wrote:
>>>> On Fri, May 24, 2024 at 09:01:45PM GMT, Mukesh Ojha wrote:
>>>>> For SCM protection, memory allocation should be physically contiguous,
>>>>> 4K aligned, and non-cacheable to avoid XPU violations. This granularity
>>>>> of protection applies from the secure world. Additionally, it's possible
>>>>> that a 32-bit secure peripheral will access memory in SoCs like
>>>>> sm8{4|5|6}50 for some remote processors. Therefore, memory allocation
>>>>> needs to be done in the lower 4 GB range. To achieve this, Linux's CMA
>>>>> pool can be used with dma_alloc APIs.
>>>>>
>>>>> However, dma_alloc APIs will fall back to the buddy pool if the requested
>>>>> size is less than or equal to PAGE_SIZE. It's also possible that the remote
>>>>> processor's metadata blob size is less than a PAGE_SIZE. Even though the
>>>>> DMA APIs align the requested memory size to PAGE_SIZE, they can still fall
>>>>> back to the buddy allocator, which may fail if `CONFIG_ZONE_{DMA|DMA32}`
>>>>> is disabled.
>>>>
>>>> Does "fail" here mean that the buddy heap returns a failure - in some
>>>> case where dma_alloc would have succeeded, or that it does give you
>>>> a PAGE_SIZE allocation which doesn't meeting your requirements?
>>>
>>> Yes, buddy will also try to allocate memory and may not get PAGE_SIZE memory
>>> in lower 4GB(for 32bit capable device) if CONFIG_ZONE_{DMA|DMA32} is
>>> disabled.
>>
>> Is that -ENOMEM or does "not get" mean that the buddy fallback will
>> provide an allocation above 4GB?
> 
> dma_alloc_coherent() returns NULL in that situation.
> 
> https://elixir.bootlin.com/linux/v6.10-rc5/source/kernel/dma/direct.c#L142
> 
> -Mukesh
> 

scm device is using DMA mask as 32b. With size <= PAGE_SIZE, call to 
dma_alloc_contiguous() at [1] will return NULL.

With DMA mask as 32b and CONFIG_ZONE_{DMA|DMA32} disabled, the allocation 
can't be guaranteed within 32b with buddy. Hence will return page as NULL.

Adding a padded page should allow allocation from CMA region.

Thanks,
Prakash

[1] https://elixir.bootlin.com/linux/v6.10-rc5/source/kernel/dma/direct.c#L131
>>
>> Regards,
>> Bjorn
>>
>>> However, DMA memory would have successful such case if
>>> padding is added to size to cross > PAGE_SIZE.
>>>
>>>>
>>>>  From this I do find the behavior of dma_alloc unintuitive, do we know if
>>>> there's a reason for the "equal to PAGE_SIZE" case you describe here?
>>>
>>> I am not a memory expert but the reason i can think of could be, <=
>>> PAGE_SIZE can anyway possible to be requested outside DMA coherent api's
>>> with kmalloc and friends api and that could be the reason it is falling
>>> back to buddy pool in DMA api.
>>>
>>> -Mukesh
> 

>