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

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Message-ID: <CA+mB=1+mrWAzBCxdMEji2MDRzHajmnxuWtz7iu96ML0DuAB3RQ@mail.gmail.com>
Date:	Mon, 20 Jan 2014 19:05:25 +0530
From:	Srikanth Thokala <sthokal@...inx.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Srikanth Thokala <sthokal@...inx.com>,
	linux-arm-kernel@...ts.infradead.org, dan.j.williams@...el.com,
	vinod.koul@...el.com, michal.simek@...inx.com,
	Grant Likely <grant.likely@...aro.org>, robh+dt@...nel.org,
	dmaengine@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma: Add Xilinx AXI Video Direct Memory Access Engine
 driver support

Hi Arnd,

Sorry for the duplication.  Ccing others.

On Mon, Jan 20, 2014 at 5:09 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Monday 20 January 2014, Srikanth Thokala wrote:
>> On Fri, Jan 17, 2014 at 9:43 PM, Arnd Bergmann <arnd@...db.de> wrote:
>> > On Thursday 16 January 2014, Srikanth Thokala wrote:
>> > I also assume that some of the properties should just go away:
>> >
>> > * xlnx,device-id should be the argument in the handle from the slave device
>>
>> We can have multiple instances of this VDMA IP configured in the FPGA and we
>> need a unique identifier for each VDMA device that is present in the FPGA.
>> This device-id dt parameter forms the filter mask for the slave devices. As an
>> example, this can be used to get a channel of specific VDMA device (assuming
>> multiple instances) using the API dma_request_channel().  Please note this
>> is an example of a slave device that doesnt have dt node.
>
> That is not a valid scenario: You should not try to support legacy
> slave devices (without DT) connected to a dma-engine that is initialized
> using DT. If you still in the progress of converting drivers to DT, you
> can use auxdata to attach additional information that can be used for
> those slave devices, but the code should be kept out of the mainline
> kernel, and I don't want to see it manifest in the bindings.

I see your point.  Thank you for the clarification.  I will fix this in my v2.

>
>> > * data width should be a property of the slave driver that is configured
>> >   through dma_slave_config(), unless you can have dma engines that only
>> >   support certain a width.
>>
>> Yes, this VDMA engine soft IP support only certain widths, which is
>> configurable during IP synthesis.
>
> But what is this property used for in that case? Surely you can't
> connect a slave device to a dmaengine if the bus width doesn't match.
> You probably have a point here, but I don't understand it yet.

There is a Data-Realignment Engine (DRE) in the IP which is only available for
data width setting of 64-bits and less.  So, we use the data-width DT parameter
to verify this condition and we update alignment shift accordingly.

Srikanth

>
>         Arnd
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