linux-kernel - Re: [PATCH v2] dma: Add Xilinx AXI Video Direct Memory Access Engine driver support

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Message-ID: <52E54849.2000208@metafoo.de>
Date:	Sun, 26 Jan 2014 18:39:21 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Vinod Koul <vinod.koul@...el.com>
CC:	Srikanth Thokala <sthokal@...inx.com>, dan.j.williams@...el.com,
	michal.simek@...inx.com, Grant Likely <grant.likely@...aro.org>,
	robh+dt@...nel.org, linux-arm-kernel@...ts.infradead.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	devicetree@...r.kernel.org, dmaengine@...r.kernel.org
Subject: Re: [PATCH v2] dma: Add Xilinx AXI Video Direct Memory Access Engine
 driver support

On 01/26/2014 02:59 PM, Vinod Koul wrote:
> On Fri, Jan 24, 2014 at 02:24:27PM +0100, Lars-Peter Clausen wrote:
>> On 01/24/2014 12:16 PM, Srikanth Thokala wrote:
>>> Hi Lars,
>>>
>>> On Thu, Jan 23, 2014 at 4:55 PM, Lars-Peter Clausen <lars@...afoo.de> wrote:
>>>> On 01/22/2014 05:52 PM, Srikanth Thokala wrote:
>>>> [...]
>>>>> +/**
>>>>> + * xilinx_vdma_device_control - Configure DMA channel of the device
>>>>> + * @dchan: DMA Channel pointer
>>>>> + * @cmd: DMA control command
>>>>> + * @arg: Channel configuration
>>>>> + *
>>>>> + * Return: '0' on success and failure value on error
>>>>> + */
>>>>> +static int xilinx_vdma_device_control(struct dma_chan *dchan,
>>>>> +                                   enum dma_ctrl_cmd cmd, unsigned long arg)
>>>>> +{
>>>>> +     struct xilinx_vdma_chan *chan = to_xilinx_chan(dchan);
>>>>> +
>>>>> +     switch (cmd) {
>>>>> +     case DMA_TERMINATE_ALL:
>>>>> +             xilinx_vdma_terminate_all(chan);
>>>>> +             return 0;
>>>>> +     case DMA_SLAVE_CONFIG:
>>>>> +             return xilinx_vdma_slave_config(chan,
>>>>> +                                     (struct xilinx_vdma_config *)arg);
>>>>
>>>> You really shouldn't be overloading the generic API with your own semantics.
>>>> DMA_SLAVE_CONFIG should take a dma_slave_config and nothing else.
>>>
>>> Ok.  The driver needs few additional configuration from the slave
>>> device like Vertical
>>> Size, Horizontal Size,  Stride etc., for the DMA transfers, in that case do you
>>> suggest me to define a separate dma_ctrl_cmd like the one FSLDMA_EXTERNAL_START
>>> defined for Freescale drivers?
>>
>> In my opinion it is not a good idea to have driver implement a generic API,
>> but at the same time let the driver have custom semantics for those API
>> calls. It's a bit like having a gpio driver that expects 23 and 42 as the
>> values passed to gpio_set_value instead of 0 and 1. It completely defeats
>> the purpose of a generic API, namely that you are able to write generic code
>> that makes use of the API without having to know about which implementation
>> API it is talking to. The dmaengine framework provides the
>> dmaengine_prep_interleaved_dma() function to setup two dimensional
>> transfers, e.g. take a look at sirf-dma.c or imx-dma.c.
> 
> The question here i think would be waht this device supports? Is the hardware
> capable of doing interleaved transfers, then would make sense.

The hardware does 2D transfers. The parameters for a transfer are height,
width and stride. That's only a subset of what interleaved transfers can be
(xt->num_frames must be one for 2d transfers). But if I remember correctly
there has been some discussion on this in the past and the result of that
discussion was that using interleaved transfers for 2D transfers is
preferred over adding a custom API for 2D transfers.

- Lars

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