On Tue, Jul 12, 2011 at 5:01 PM, Raju, Sundaram [off-list ref] wrote:

quoted

-----Original Message-----
From: Jassi Brar [mailto:jassisinghbrar@gmail.com]
Sent: Tuesday, July 12, 2011 4:51 PM
To: Linus Walleij
Cc: Raju, Sundaram; linux-arm-kernel@lists.infradead.org; linux-
kernel@vger.kernel.org; davinci-linux-open-source@linux.davincidsp.com;
linux@arm.linux.org.uk; dan.j.williams@intel.com; linux-omap@vger.kernel.org
Subject: Re: [PATCH] dmaengine: add dma_ctrl_cmd to pass buffer stride
configuration

On Tue, Jul 12, 2011 at 3:33 PM, Linus Walleij [off-list ref]
wrote:

quoted

On Tue, Jul 12, 2011 at 6:17 AM, Jassi Brar [off-list ref]

wrote:

quoted

quoted

1) Striding, in one form or other, is supported by other DMACs as well.
  The number will only increase in future.
  Are we to add  <VENDOR>_DMA_STRIDE_CONFIG for each case ?

If we are sure about this and striding will work in a similar way on all
then let's have the enum named DMA_STRIDE_CONFIG and move the
passed-in struct to <linux/dmaengine.h) then?

Would that be:

struct dma_stride_config {
   u32 read_bytes;
   u32 skip_bytes;
};

Or something more complex?

Well, I am not sure if striding needs any special treatment at all.
Why not have client drivers prepare and submit sg-list.
Let the DMAC drivers interpret/parse the sg-list and program it
as strides if the h/w supports it.
If anything, we should make preparation and submission of sg-list
as efficient as possible.

Jassi,

sg_lists describe only a bunch of disjoint buffers. But what if the
DMAC can skip and read the bytes within each of the buffers in
the sg_list? (like TI EDMAC and TI SDMAC)
How can that information be passed to the offload
engine driver from the client?

OK, I overlooked.
We do need something new to handle these ultra-fine-grained sg-lists.
But still we shouldn't add SoC specific API to the common sub-systems.

Maybe a new api to pass fixed-format variable-length encoded message
to the DMAC drivers?
Which could be interpreted by DMAC drivers to extract all the needed xfer
parameters from the 'header' section and instructions to program the xfers
in the DMAC from the variable length body of the 'message' buffer.
It might sound complicated but we can have helpers to make the job easy.
Btw, the regular single/sg-list xfers could also be expressed by this method.