Re: [PATCH v4 13/14] dt-bindings: of: Add restricted DMA pool

[Florian Fainelli <f.fainelli@gmail.com>]

On 3/10/2021 1:40 PM, Rob Herring wrote:
> On Wed, Mar 10, 2021 at 9:08 AM Will Deacon <will@kernel.org> wrote:
>>
>> Hi Claire,
>>
>> On Tue, Feb 09, 2021 at 02:21:30PM +0800, Claire Chang wrote:
>>> Introduce the new compatible string, restricted-dma-pool, for restricted
>>> DMA. One can specify the address and length of the restricted DMA memory
>>> region by restricted-dma-pool in the reserved-memory node.
>>>
>>> Signed-off-by: Claire Chang <tientzu@chromium.org>
>>> ---
>>>  .../reserved-memory/reserved-memory.txt       | 24 +++++++++++++++++++
>>>  1 file changed, 24 insertions(+)
>>>
>>> diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
>>> index e8d3096d922c..fc9a12c2f679 100644
>>> --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
>>> +++ b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
>>> @@ -51,6 +51,20 @@ compatible (optional) - standard definition
>>>            used as a shared pool of DMA buffers for a set of devices. It can
>>>            be used by an operating system to instantiate the necessary pool
>>>            management subsystem if necessary.
>>> +        - restricted-dma-pool: This indicates a region of memory meant to be
>>> +          used as a pool of restricted DMA buffers for a set of devices. The
>>> +          memory region would be the only region accessible to those devices.
>>> +          When using this, the no-map and reusable properties must not be set,
>>> +          so the operating system can create a virtual mapping that will be used
>>> +          for synchronization. The main purpose for restricted DMA is to
>>> +          mitigate the lack of DMA access control on systems without an IOMMU,
>>> +          which could result in the DMA accessing the system memory at
>>> +          unexpected times and/or unexpected addresses, possibly leading to data
>>> +          leakage or corruption. The feature on its own provides a basic level
>>> +          of protection against the DMA overwriting buffer contents at
>>> +          unexpected times. However, to protect against general data leakage and
>>> +          system memory corruption, the system needs to provide way to lock down
>>> +          the memory access, e.g., MPU.
>>
>> As far as I can tell, these pools work with both static allocations (which
>> seem to match your use-case where firmware has preconfigured the DMA ranges)
>> but also with dynamic allocations where a 'size' property is present instead
>> of the 'reg' property and the kernel is responsible for allocating the
>> reservation during boot. Am I right and, if so, is that deliberate?
> 
> I believe so. I'm not keen on having size only reservations in DT.
> Yes, we allowed that already, but that's back from the days of needing
> large CMA carveouts to be reserved early in boot. I've read that the
> kernel is much better now at contiguous allocations, so do we really
> need this in DT anymore?

I would say yes, there can be a number of times where you want to semi
statically partition your physical memory and their reserved regions. Be
it to pack everything together under the same protection rules or
because you need to allocate memory from a particular address range in
say a non-uniform memory controller architecture where address windows
have different scheduling algorithms.
-- 
Florian