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Subject: Re: [RFC 2/6] dmaengine: xilinx_dma: Pass AXI4-Stream control words
 to netdev dma client
To:     Lars-Peter Clausen <lars@metafoo.de>,
        Radhey Shyam Pandey <radheys@xilinx.com>,
        Vinod Koul <vinod.koul@intel.com>
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        "dan.j.williams@intel.com" <dan.j.williams@intel.com>,
        Appana Durga Kedareswara Rao <appanad@xilinx.com>,
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From:   Peter Ujfalusi <peter.ujfalusi@ti.com>
Message-ID: <a6ac9ca8-91b4-c7b1-0f0c-c04dd10048e2@ti.com>
Date:   Wed, 18 Apr 2018 09:31:39 +0300
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On 2018-04-17 18:54, Lars-Peter Clausen wrote:
> On 04/17/2018 04:53 PM, Peter Ujfalusi wrote:
>> On 2018-04-17 16:58, Lars-Peter Clausen wrote:
>>>>> There are two options.
>>>>>
>>>>> Either you extend the generic interfaces so it can cover your usecase in a
>>>>> generic way. E.g. the ability to attach meta data to transfer.
>>>>
>>>> Fwiw I have this patch as part of a bigger work to achieve similar results:
>>>
>>> That's good stuff. Is this in a public tree somewhere?
>>
>> Not atm. I can not send the user of the new API and I did not wanted to
>> send something like this out of the blue w/o context.
>>
>> But as it is a generic patch, I can send it as well. The only thing is
>> that the need for the memcpy, so I might end up with
>> ptr = get_metadata_ptr(desc, &size); /* size: in RX the valid size */
>>
>> and set_metadata_size(); /* in TX to tell how the client placed */
>>
>> Or something like that, the attach_metadata() as it is works just fine,
>> but high throughput might not like the memcpy.
>>
> 
> In the most abstracted way I'd say metadata and data are two different data
> streams that are correlated and send/received at the same time.

In my case the meatdata is sideband information or parameters for/from
the remote end. Like timestamp, algorithm parameters, keys, etc.

It is tight to the data payload, but it is not part of it.

But the API should be generic enough to cover other use cases where
clients need to provide additional information.
For me, the metadata is part of the descriptor we give and receive back
from the DMA, others might have sideband channel to send that.

For metadata handling we could have:

struct dma_desc_metadata_ops {
   /* To give a buffer for the DMA with the metadata, as it was in my
    * original patch
    */
   int (*desc_attach_metadata)(struct dma_async_tx_descriptor *desc,
                               void *data, size_t len);

   void *(*desc_get_metadata_ptr)(struct dma_async_tx_descriptor *desc,
                                  size_t *payload_len, size_t *max_len);
   int (*desc_set_payload_len)(struct dma_async_tx_descriptor *desc,
                              size_t payload_len);
};

Probably a simple flag variable to indicate which of the two modes are
supported:
1. Client provided metadata buffer handling
Clients provide the buffer via desc_attach_metadata(), the DMA driver
will do whatever it needs to do, copy it in place, send it differently,
use parameters.
In RX the received metadata is going to be placed to the provided buffer.
2. Ability to give the metadata pointer to user to work on it.
In TX, clients can use desc_get_metadata_ptr() to get the pointer,
current payload size and maximum size of the metadata and can work
directly on the buffer to place the data. Then desc_set_payload_len() to
let the DMA know how much data is actually placed there.
In RX, desc_get_metadata_ptr() will give the user the pointer and the
payload size so it can process that information correctly.

DMA driver can implement either or both, but clients must only use
either 1 or 2 to work with the metadata.


> Think multi-planar transfer, like for audio when the right and left channel
> are in separate buffers and not interleaved. Or video with different
> color/luminance components in separate buffers. This is something that is at
> the moment not covered by the dmaengine API either.

Hrm, true, but it is hardly the metadata use case. It is more like
different DMA transfer type.

>>>>> Or you can implement a interface that is specific to your DMA controller and
>>>>> any client using this interface knows it is talking to your DMA controller.
>>>>
>>>> Hrm, so we can have DMA driver specific calls? The reason why TI's keystone 2
>>>> navigator DMA support was rejected that it was introducing NAV specific calls
>>>> for clients to configure features not yet supported by the framework.
>>>
>>> In my opinion it is OK, somebody else might have different ideas. I mean it
>>> is not nice, but it is better than the alternative of overloading the
>>> generic API with driver specific semantics or introducing some kind of IOCTL
>>> catch all callback.
>>
>> True, but the generic API can be extended as well to cover new grounds,
>> features. Like this metadata thing.
>>
>>> If there is tight coupling between the DMA core and client and there is no
>>> intention of using a generic client the best solution might even be to no
>>> use DMAengine at all.
>>
>> This is how the knav stuff ended up. Well it is only used by networking
>> atm, so it is 'fine' to have custom API, but it is not portable.
> 
> I totally agree generic APIs are better, but not everybody has the resources
> to rewrite the whole framework just because they want to do this tiny thing
> that isn't covered by the framework yet. In that case it is better to go
> with a custom API (that might evolve into a generic API), rather than
> overloading the generic API and putting a strain on everybody who works on
> the generic API.

At some point a threshold is reached when the burden of maintaining a
custom API is more costly than investing on the extension of the framework.
What happens when an existing driver (using DMAengine API) need to be
supported on platform where only custom DMA code is available or if you
want to migrate to a new DMA from the custom API the drier is wired for?
It is just plain pain.
At the end what we want to do with DMA: move data from opne place to
another (in oversimplified view).

The framework can be extended to cover new features w/o much impact on
generality or drivers do not need the feature, like the metadata. The
impact is minimal for drivers who don't care.

If there is interest I can send the core patches for the metadata in
couple of days (need to update the documentation as well) and test the
new give me the metadata pointer support.
Unfortunately I can not send atm the DMA driver itself which is using this.
If this helps others to move ahead, I'm fine spending extra time to get
it in a good shape for upstream.

- Péter

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