[RFC 2/6] dmaengine: xilinx_dma: Pass AXI4-Stream control words to netdev dma client

[Lars-Peter Clausen]

On 04/17/2018 01:43 PM, Radhey Shyam Pandey wrote:
> Hi Vinod,
> 
>> -----Original Message-----
>> From: Vinod Koul [mailto:vinod.koul at intel.com]
>> Sent: Wednesday, April 11, 2018 2:39 PM
>> To: Radhey Shyam Pandey <radheys at xilinx.com>
>> Cc: dan.j.williams at intel.com; michal.simek at xilinx.com; Appana Durga
>> Kedareswara Rao <appanad at xilinx.com>; Radhey Shyam Pandey
>> <radheys at xilinx.com>; lars at metafoo.de; dmaengine at vger.kernel.org;
>> linux-arm-kernel at lists.infradead.org; linux-kernel at vger.kernel.org
>> Subject: Re: [RFC 2/6] dmaengine: xilinx_dma: Pass AXI4-Stream control
>> words to netdev dma client
>>
>> On Mon, Apr 02, 2018 at 04:09:02PM +0530, Radhey Shyam Pandey wrote:
>>
>>> +
>>> +		if (chan->xdev->has_axieth_connected) {
>>> +			seg = list_first_entry(&desc->segments,
>>> +					struct xilinx_axidma_tx_segment,
>> node);
>>> +			if (cb.callback_param) {
>>> +				app_w = (u32 *) cb.callback_param;
>>
>> why are you interpreting callback_param? This is plainly wrong.
>> we do not know what is the interpretation of callback_param and it is
>> internal to submitter.
> In design, if AXI DMA is connected to AXI Ethernet IP there are certain
> AXI4-Stream Status fields (RX) that we need to pass to ethernet driver
> along with data buffer. An example includes: checksum fields, packet
> length etc. To pass these control words there is a structure defined
> between dmaengine and client. Before calling the client callback
> stream control words are copied to dma client callback_param struct
> (only if axieth is connected).
> 
> I understand it's not an ideal way and we shouldn't be interpreting
> callback_param but couldn't find any better alternative of passing
> custom information from dmaengine to client driver and still be 
> aligned to the framework.
> 
>>
>> What exactly is the problem you are trying to solve?
> As mentioned above we need to pass AXI4-stream words(custom
> data) from dmaengine driver to dma client driver(ethernet) for
> each DMA descriptor. Current solution populates callback_param
> struct (only if axieth is connected). Please let me know if there is
> an alternate solution. 

The standard interfaces need to work in a way that a client using them does
not have to know to which DMA controller it is connected. In a similar way
the DMA controller shouldn't have any client specific logic for the generic
interfaces. Otherwise there is no point of having a generic interface.

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.

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.