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

[Mukesh Ojha <quic_mojha@quicinc.com>]

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

> 
> 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