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

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Message-ID: <CA+mB=1LyX0_VVXZQ-=m3gR2RBeumLrnPTUW3j+Zm1nj7a4exHg@mail.gmail.com>
Date:	Fri, 31 Jan 2014 12:22:52 +0530
From:	Srikanth Thokala <sthokal@...inx.com>
To:	Srikanth Thokala <sthokal@...inx.com>
Cc:	Vinod Koul <vinod.koul@...el.com>,
	Lars-Peter Clausen <lars@...afoo.de>, 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

Hi Vinod,

On Mon, Jan 27, 2014 at 4:36 PM, Srikanth Thokala <sthokal@...inx.com> wrote:
> Hi Vinod,
>
> On Sun, Jan 26, 2014 at 7:29 PM, Vinod Koul <vinod.koul@...el.com> 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.
>>
>> 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
>
> There are many configuration parameters which are specific to IP and I
> would like to
> give an overview of some of parameteres here:
>
> 1) Park Mode ('cfg->park'): In Park mode, engine will park on frame
> referenced by
>     'cfg->park_frm', so user will have control on each frame in this mode.
>
> 2) Interrupt Coalesce ('cfg->coalesce'):  Used for setting interrupt
> threshold. This value
>    determines the number of frame buffers to process. To use this feature,
>    'cfg->frm_cnt_en' should be set.
>
> 3) Frame Synchronization Source ('cfg->ext_fsync'):  Can be an
> external/internal frame
>     synchronization source. Used to synchronize one channel (MM2S/S2MM) with
>     another (S2MM/MM2S) channel.
>
> 4) Genlock Synchronization ('cfg->genlock'): Used to avoid mismatch rate between
>     master and slave.  In master mode (cfg->master), frames are not dropped and
>     slave can drop frames to adjust to master frame rate.
>
> And in future, this Engine being a soft IP, we could expect some more additional
> parameters.  Isn't a good idea to have a private member in dma_slave_config for
> sharing additional configuration between slave device and dma engine? Or a new
> dma_ctrl_cmd like FSLDMA_EXTERNAL_START?


Ping?

>
> Srikanth
>
>>
>> --
>> ~Vinod
>> --/EX
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