linux-kernel - Re: [PATCH] [RFC] dmaengine: add fifo

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Date:   Thu, 2 May 2019 17:55:06 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Sameer Pujar <spujar@...dia.com>
Cc:     dan.j.williams@...el.com, tiwai@...e.com, jonathanh@...dia.com,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RFC] dmaengine: add fifo_size member

On 02-05-19, 16:23, Sameer Pujar wrote:
> 
> On 5/2/2019 11:34 AM, Vinod Koul wrote:
> > On 30-04-19, 17:00, Sameer Pujar wrote:
> > > During the DMA transfers from memory to I/O, it was observed that transfers
> > > were inconsistent and resulted in glitches for audio playback. It happened
> > > because fifo size on DMA did not match with slave channel configuration.
> > > 
> > > currently 'dma_slave_config' structure does not have a field for fifo size.
> > > Hence the platform pcm driver cannot pass the fifo size as a slave_config.
> > > Note that 'snd_dmaengine_dai_dma_data' structure has fifo_size field which
> > > cannot be used to pass the size info. This patch introduces fifo_size field
> > > and the same can be populated on slave side. Users can set required size
> > > for slave peripheral (multiple channels can be independently running with
> > > different fifo sizes) and the corresponding sizes are programmed through
> > > dma_slave_config on DMA side.
> > FIFO size is a hardware property not sure why you would want an
> > interface to program that?
> > 
> > On mismatch, I guess you need to take care of src/dst_maxburst..
> Yes, FIFO size is a HW property. But it is SW configurable(atleast in my
> case) on
> slave side and can be set to different sizes. The src/dst_maxburst is

Are you sure, have you talked to HW folks on that? IIUC you are
programming the data to be used in FIFO not the FIFO length!

> programmed
> for specific values, I think this depends on few factors related to
> bandwidth
> needs of client, DMA needs of the system etc.,

Precisely

> In such cases how does DMA know the actual FIFO depth of slave peripheral?

Why should DMA know? Its job is to push/pull data as configured by
peripheral driver. The peripheral driver knows and configures DMA
accordingly.

 
> > > Request for feedback/suggestions.
> > > 
> > > Signed-off-by: Sameer Pujar <spujar@...dia.com>
> > > ---
> > >   include/linux/dmaengine.h | 3 +++
> > >   1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> > > index d49ec5c..9ec198b 100644
> > > --- a/include/linux/dmaengine.h
> > > +++ b/include/linux/dmaengine.h
> > > @@ -351,6 +351,8 @@ enum dma_slave_buswidth {
> > >    * @slave_id: Slave requester id. Only valid for slave channels. The dma
> > >    * slave peripheral will have unique id as dma requester which need to be
> > >    * pass as slave config.
> > > + * @fifo_size: Fifo size value. The dma slave peripheral can configure required
> > > + * fifo size and the same needs to be passed as slave config.
> > >    *
> > >    * This struct is passed in as configuration data to a DMA engine
> > >    * in order to set up a certain channel for DMA transport at runtime.
> > > @@ -376,6 +378,7 @@ struct dma_slave_config {
> > >   	u32 dst_port_window_size;
> > >   	bool device_fc;
> > >   	unsigned int slave_id;
> > > +	u32 fifo_size;
> > >   };
> > >   /**
> > > -- 
> > > 2.7.4

-- 
~Vinod