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

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Date:   Tue, 17 Apr 2018 14:54:48 +0200
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Radhey Shyam Pandey <radheys@...inx.com>,
        Vinod Koul <vinod.koul@...el.com>
Cc:     "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "michal.simek@...inx.com" <michal.simek@...inx.com>,
        Appana Durga Kedareswara Rao <appanad@...inx.com>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/6] dmaengine: xilinx_dma: Pass AXI4-Stream control words
 to netdev dma client

On 04/17/2018 01:43 PM, Radhey Shyam Pandey wrote:
> Hi Vinod,
> 
>> -----Original Message-----
>> From: Vinod Koul [mailto:vinod.koul@...el.com]
>> Sent: Wednesday, April 11, 2018 2:39 PM
>> To: Radhey Shyam Pandey <radheys@...inx.com>
>> Cc: dan.j.williams@...el.com; michal.simek@...inx.com; Appana Durga
>> Kedareswara Rao <appanad@...inx.com>; Radhey Shyam Pandey
>> <radheys@...inx.com>; lars@...afoo.de; dmaengine@...r.kernel.org;
>> linux-arm-kernel@...ts.infradead.org; linux-kernel@...r.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.