linux-kernel - Re: [PATCH 1/3] dt-bindings: dma: Add documentation for DMA domains

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Date:   Mon, 16 Sep 2019 14:21:50 +0300
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     Rob Herring <robh@...nel.org>
CC:     <vkoul@...nel.org>, <dmaengine@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <dan.j.williams@...el.com>,
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH 1/3] dt-bindings: dma: Add documentation for DMA domains



On 13/09/2019 17.36, Rob Herring wrote:
> On Tue, Sep 10, 2019 at 02:50:35PM +0300, Peter Ujfalusi wrote:
>> On systems where multiple DMA controllers available, non Slave (for example
>> memcpy operation) users can not be described in DT as there is no device
>> involved from the DMA controller's point of view, DMA binding is not usable.
>> However in these systems still a peripheral might need to be serviced by or
>> it is better to serviced by specific DMA controller.
>> When a memcpy is used to/from a memory mapped region for example a DMA in the
>> same domain can perform better.
>> For generic software modules doing mem 2 mem operations it also matter that
>> they will get a channel from a controller which is faster in DDR to DDR mode
>> rather then from the first controller happen to be loaded.
>>
>> This property is inherited, so it may be specified in a device node or in any
>> of its parent nodes.
> 
> If a device needs mem2mem dma, I think we should just use the existing 
> dma binding. The provider will need a way to define cell values which 
> mean mem2mem.

But isn't it going to be an abuse of the binding? Each DMA controller
would hack this in different ways, probably using out of range DMA
request/trigger number or if they have direction in the binding or some
other parameter would be set to something invalid...

> For generic s/w, it should be able to query the dma speed or get a 
> preferred one IMO. It's not a DT problem.
> 
> We measure memcpy speeds at boot time to select the fastest 
> implementation for a chip, why not do that for mem2mem DMA?

It would make an impact on boot time since the tests would need to be
done with a large enough copy to be able to see clearly which one is faster.

Also we should be able to handle different probing orders:
client1 should have mem2mem channel from dma2.

- dma1 probes
- client1 probes and asks for a mem2mem channel
- dma2 probes

Here client1 should deffer until dma2 is probed.

Probably the property should be dma-mem2mem-domain to be more precise on
it's purpose and avoid confusion?

> 
>>
>> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
>> ---
>>  .../devicetree/bindings/dma/dma-domain.yaml   | 88 +++++++++++++++++++
>>  1 file changed, 88 insertions(+)
>>  create mode 100644 Documentation/devicetree/bindings/dma/dma-domain.yaml
> 
> Note that you have several errors in your schema. Run 'make dt_bindings_check'.

That does not do anything on my system, but git dt-doc-validate running
via https://github.com/robherring/yaml-bindings.git.

- Péter

Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
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