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

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Date:	Tue, 28 Jan 2014 08:39:16 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Lars-Peter Clausen <lars@...afoo.de>
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 Sun, Jan 26, 2014 at 06:39:21PM +0100, Lars-Peter Clausen wrote:
> 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.
Yup that would be my recomendation. Moving this driver to interleaved API seems
right to me

-- 
~Vinod
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