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

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Date: Sun, 23 Jun 2024 19:25:23 -0500
From: Bjorn Andersson <andersson@...nel.org>
To: Mukesh Ojha <quic_mojha@...cinc.com>
Cc: konrad.dybcio@...aro.org, linux-arm-msm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] firmware: qcom_scm: Add a padded page to ensure DMA
 memory from lower 4GB

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?

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