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

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Date:	Sun, 26 Jan 2014 19:29:33 +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 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.

While we do try to get users use dma_slave_config, but there will always be
someone who have specfic params. If we can generalize then we might want to add
to the dma_slave_config as well

--
~Vinod
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