linux-kernel - Re: [PATCH v3 2/2] dma: Add Xilinx AXI Direct Memory Access Engine driver support

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Date:	Tue, 9 Sep 2014 00:52:16 +0530
From:	Srikanth Thokala <sthokal@...inx.com>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	Srikanth Thokala <sthokal@...inx.com>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	Michal Simek <michal.simek@...inx.com>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Levente Kurusa <levex@...ux.com>,
	Jassi Brar <jassisinghbrar@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dmaengine@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, devicetree@...r.kernel.org,
	anirudh@...inx.com, svemula@...inx.com
Subject: Re: [PATCH v3 2/2] dma: Add Xilinx AXI Direct Memory Access Engine
 driver support

Hi Vinod,

On Thu, Sep 4, 2014 at 12:06 PM, Vinod Koul <vinod.koul@...el.com> wrote:
> On Wed, Sep 03, 2014 at 12:17:43PM +0530, Srikanth Thokala wrote:
>> Hi Vinod,
>>
>> Apologies for the delay.
>>
>> On Tue, Aug 19, 2014 at 10:33 PM, Vinod Koul <vinod.koul@...el.com> wrote:
>> > On Mon, Jul 28, 2014 at 05:47:49PM +0530, Srikanth Thokala wrote:
>> >> +struct xilinx_dma_chan {
>> >> +     struct xilinx_dma_device *xdev;
>> >> +     u32 ctrl_offset;
>> >> +     spinlock_t lock;
>> >> +     struct list_head pending_list;
>> >> +     struct xilinx_dma_tx_descriptor *active_desc;
>> >> +     struct xilinx_dma_tx_descriptor *allocated_desc;
>> >> +     struct list_head done_list;
>> >> +     struct list_head free_seg_list;
>> >> +     struct dma_chan common;
>> >> +     struct xilinx_dma_tx_segment *seg_v;
>> >> +     dma_addr_t seg_p;
>> >> +     struct device *dev;
>> >> +     int irq;
>> >> +     int id;
>> >> +     enum dma_transfer_direction direction;
>> > This looks suspect. Why should channel have direction, for a descriptor it
>> > makes sense though.
>>
>> The channel only supports transfers in one direction. Either from memory to
>> peripheral or from peripheral to memory, that's fixed and can't be changed
>> at runtime.  So, the driver needs to know which direction the channel supports
>> and hence it can reject transfers with the wrong direction.
> But you already have this information in descriptor so why duplicate?

Our descriptor doesn't have the information of channel direction, we have this
information only in the chan structure (struct xilinx_dma_chan) which
is populated
while parsing the DT.  So, we use this information to ensure if we are servicing
proper channel (in prep_slave_sg call).

FYI: We also need the direction of the channel so that we can select SRC/DST
register address in the issue_pending() call.

Could you please elaborate, if I am missing something?

>
>> >> +/**
>> >> + * xilinx_dma_channel_set_config - Configure DMA channel
>> >> + * @dchan: DMA channel
>> >> + * @cfg: DMA device configuration pointer
>> >> + *
>> >> + * Return: '0' on success and failure value on error
>> >> + */
>> >> +int xilinx_dma_channel_set_config(struct dma_chan *dchan,
>> >> +                               struct xilinx_dma_config *cfg)
>> >> +{
>> >> +     struct xilinx_dma_chan *chan = to_xilinx_chan(dchan);
>> >> +     u32 reg = dma_ctrl_read(chan, XILINX_DMA_REG_CONTROL);
>> >> +
>> >> +     if (cfg->reset)
>> >> +             return xilinx_dma_reset(chan);
>> >> +
>> >> +     if (cfg->coalesc <= XILINX_DMA_CR_COALESCE_MAX)
>> >> +             reg |= cfg->coalesc << XILINX_DMA_CR_COALESCE_SHIFT;
>> >> +
>> >> +     if (cfg->delay <= XILINX_DMA_CR_DELAY_MAX)
>> >> +             reg |= cfg->delay << XILINX_DMA_CR_DELAY_SHIFT;
>> >> +
>> >> +     dma_ctrl_write(chan, XILINX_DMA_REG_CONTROL, reg);
>> > You aren't checking if a transaction is already running on this channel.
>> > Also don't you need other slave parameters, I see you have removed the
>> > dma_slave_config entirely
>>
>> All the parameters that are required for this DMA engine are provided by
>> SG list, so I don't see the need of dma_slave_config.  There are some
>> specific IP parameters that are not part of dma_slave_config and these
>> are being set by 'dma_channel_set_config' which is exported to slave
>> drivers.
> And that means you dont have any common parameters between dma_slave_config
> right?

Yes, true.

Thanks,
Srikanth

>
> --
> ~Vinod
>
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