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

[Srikanth Thokala]

Hi Vinod,

On Tue, Sep 9, 2014 at 9:27 PM, Vinod Koul <vinod.koul at intel.com> wrote:
> On Tue, Sep 09, 2014 at 12:52:16AM +0530, Srikanth Thokala wrote:
>> Hi Vinod,
>>
>> On Thu, Sep 4, 2014 at 12:06 PM, Vinod Koul <vinod.koul at intel.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 at intel.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).
> One of the argument of prep_slave_sg call is direction. You need to use that
> and store in in your descriptor for using it afterwards
>
>> FYI: We also need the direction of the channel so that we can select SRC/DST
>> register address in the issue_pending() call.
> And you have the descriptor there as well so can use it

Ok, got it. Thanks.  I will send v4 with the changes mentioned.

- Srikanth

>
> --
> ~Vinod
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