linux-kernel - Re: [PATCH 4/5] dmaengine: sprd: Add Spreadtrum DMA configuration

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Date:   Fri, 13 Apr 2018 15:41:41 +0530
From:   Vinod Koul <vinod.koul@...el.com>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Eric Long <eric.long@...eadtrum.com>,
        Mark Brown <broonie@...nel.org>, dmaengine@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] dmaengine: sprd: Add Spreadtrum DMA configuration

On Fri, Apr 13, 2018 at 02:41:48PM +0800, Baolin Wang wrote:
> On 13 April 2018 at 14:36, Vinod Koul <vinod.koul@...el.com> wrote:
> > On Fri, Apr 13, 2018 at 02:17:34PM +0800, Baolin Wang wrote:
> >
> >> > Agreed, users only care about grabbing a channel, setting a descriptor and
> >> > submitting that.
> >> >
> >> > I think you need to go back and think about this a bit, please do go thru
> >> > dmaengine documentation and see other driver examples.
> >> >
> >> > We don't typically expose these to users, they give us a transfer and we set
> >> > that up in hardware for efficient. Its DMA so people expect us to use fastest
> >> > mechanism available.
> >>
> >> But there are some configuration are really special for Spreadtrum
> >> DMA, and must need user to specify how to configure, especially some
> >> scenarios of audio. So I wander if we can add one pointer for
> >> 'dma_slave_config' to expand some special DMA configuration
> >> requirements, like:
> >>
> >> struct dma_slave_config {
> >>     ......
> >>     unsigned int slave_id;
> >>     void *platform_data;
> >> };
> >>
> >> So if some DMA has some special configuration (such as Spreadtrum
> >> DMA), they can user this platform_data pointer. Like xilinx DMA, they
> >> also have some special configuration.
> >
> > Well we all think our HW is special and needs some additional stuff, most of
> > the cases turns out not to be the case.
> >
> > Can you explain how audio in this case additional configuration...
> >
> 
> Beside the general configuration, our audio driver will configure the
> fragment length, block length, maybe transaction length, and they must
> specify the request type and interrupt type, these are what we want to
> export for users.

First doesn't it use sound dmaengine library, it should :)

Second, I think you should calculate the lengths based on given input. Audio
is circular buffer so you shall create a circular linked list and submit.
See how other driver implement circular prepare callback

-- 
~Vinod