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Subject: Re: [PATCH 1/3] dt-bindings: dma: Add documentation for DMA domains
From:   Peter Ujfalusi <peter.ujfalusi@ti.com>
To:     Rob Herring <robh@kernel.org>
CC:     <vkoul@kernel.org>, <dmaengine@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <dan.j.williams@intel.com>,
        <devicetree@vger.kernel.org>
References: <20190910115037.23539-1-peter.ujfalusi@ti.com>
 <20190910115037.23539-2-peter.ujfalusi@ti.com>
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 <82254a3e-12fe-14d8-d49a-6627dd1d3559@ti.com>
Message-ID: <ed0de4d9-961a-3dcc-5865-d54b298ac309@ti.com>
Date:   Tue, 24 Sep 2019 16:56:32 +0300
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On 16/09/2019 14.21, Peter Ujfalusi wrote:
> 
> 
> 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?

Is it OK if I go with dma-mem2mem-domain or dma-mem2mem-controller for
v2, but keeping the logic and approach intact?

Regards,
- Péter

> 
>>
>>>
>>> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.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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> 

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