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

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Date:   Sat, 4 May 2019 15:53:04 +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, 18:59, Sameer Pujar wrote:
> 
> On 5/2/2019 5:55 PM, Vinod Koul wrote:
> > 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!
> Yes, I mentioned about FIFO length.
> 
> 1. MAX FIFO size is fixed in HW. But there is a way to limit the usage per
> channel
>    in multiples of 64 bytes.
> 2. Having a separate member would give independent control over MAX BURST
> SIZE and
>    FIFO SIZE.
> > 
> > > 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.
> I am not sure if there is any HW logic that mandates DMA to know the size
> of configured FIFO depth on slave side. I will speak to HW folks and
> would update here.

I still do not comprehend why dma would care about slave side
configuration. In the absence of patch which uses this I am not sure
what you are trying to do...

> > > > > 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