[RFC PATCH 2/4] ASoC: soc-generic-dmaengine-pcm: Add custom prepare and submit function

[Shevchenko, Andriy]

On Wed, Nov 18, 2020 at 02:34:22AM +0200, Sia, Jee Heng wrote:
> > From: Andy Shevchenko <andriy.shevchenko at intel.com>
> > Sent: 17 November 2020 11:51 PM
> > On Tue, Nov 17, 2020 at 04:03:48PM +0800, Michael Sit Wei Hong wrote:
> > > Enabling custom prepare and submit function to overcome DMA limitation.
> > >
> > > In the Intel KeemBay solution, the DW AXI-based DMA has a limitation
> > > on the number of DMA blocks per transfer. In the case of 16 bit audio
> > > ASoC would allocate blocks exceeding the DMA block limitation.
> >
> > I'm still not sure the hardware has such a limitation.
> >
> > The Synopsys IP supports linked list (LLI) transfers and I hardly can imagine the
> > list has any limitation. Even though, one can always emulate LLI in software how
> > it's done in the DesignWare AHB DMA driver.
> >
> > I have briefly read chapter 4.6 "AXI_DMA" of Keem Bay TRM and didn't find any
> > errata or limits like this.
> [>>] Intel KeemBay datasheet can be found at below link:
> https://www.intel.com/content/www/us/en/secure/design/confidential/products-and-solutions/processors-and-chipsets/keem-bay/technical-library.html?grouping=EMT_Content%20Type&sort=title:asc
> Pg783, "Programmable transfer length (block length), max 1024". Sub-list can't exceed 1024 blocks.
> BTW, I think the 16bits audio could be a confusion because it is not about the number of bits, but rather the constraint of the block length. Base on my understanding, Audio needs a period larger than 10ms, regardless of the bit depth.
> The questions here are:
> 1. Should DMAEngine expose a new API to constraint the block_length (instead of size in bytes)?
> 2. Should DMA client re-split the linked-list before passing the linked-list to DMAEngine.
> 3. Should DMA controller driver re-split the linked-list before execution.

Since we have DMA slave capabilities, the consumer may ask for them and prepare
the SG list accordingly.

Above limitation is a block size (value of 1023 is a maximum, meaning 1024
maximum items in the block of given transfer width). So, like DesignWare DMA
(AHB) has up to 4095 (but I saw hardware with 2047) or iDMA 32- and 64-bit have
131071. There is no limitation for amount of blocks of given maximum length in
the LLI!

No hacks are needed, really.

-- 
With Best Regards,
Andy Shevchenko