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

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Message-ID: <3368d1e1-0d7f-f602-5b96-a978fcf4d91b@nvidia.com>
Date:   Thu, 2 May 2019 18:59:09 +0530
From:   Sameer Pujar <spujar@...dia.com>
To:     Vinod Koul <vkoul@...nel.org>
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 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.
>   
>>>> 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