Re: [linux-sunxi] [PATCH 05/16] ASoc: sun4i-i2s: Add 20 and 24 bit support

Samuel Holland
2020-09-03T02:24:06Z

On 9/2/20 1:10 PM, Jernej Škrabec wrote:
> Hi Samuel!
>
> Dne petek, 10. julij 2020 ob 07:44:51 CEST je Samuel Holland napisal(a):
>> On 7/4/20 6:38 AM, Clément Péron wrote:
>>> From: Marcus Cooper <codekipper@gmail.com>
>>>
>>> Extend the functionality of the driver to include support of 20 and
>>> 24 bits per sample.
>>>
>>> Signed-off-by: Marcus Cooper <codekipper@gmail.com>
>>> Signed-off-by: Clément Péron <peron.clem@gmail.com>
>>> ---
>>>
>>> sound/soc/sunxi/sun4i-i2s.c | 11 +++++++++--
>>> 1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/sound/soc/sunxi/sun4i-i2s.c b/sound/soc/sunxi/sun4i-i2s.c
>>> index f78167e152ce..bc7f9343bc7a 100644
>>> --- a/sound/soc/sunxi/sun4i-i2s.c
>>> +++ b/sound/soc/sunxi/sun4i-i2s.c
>>> @@ -577,6 +577,9 @@ static int sun4i_i2s_hw_params(struct
>>> snd_pcm_substream *substream,>
>>> case 16:
>>> width = DMA_SLAVE_BUSWIDTH_2_BYTES;
>>> break;
>>>
>>> + case 32:
>>> + width = DMA_SLAVE_BUSWIDTH_4_BYTES;
>>> + break;
>>
>> This breaks the sun4i variants, because sun4i_i2s_get_wss returns 4 for a 32
>> bit width, but it needs to return 3.
>
> I'm not sure what has WSS with physical width and DMA?

This is the change where creating a S24_LE stream no longer fails with -EINVAL.
So this is the change where userspace stops downsampling 24-bit audio sources.
So this is the change where playback of 24-bit audio sources breaks, because WSS
is programmed wrong.

>> As a side note, I wonder why we use the physical width (the spacing between
>> samples in RAM) to drive the slot width. S24_LE takes up 4 bytes per sample
>> in RAM, which we need for DMA. But I don't see why we would want to
>> transmit the padding over the wire. I would expect it to be transmitted the
>> same as S24_3LE (which has no padding). It did not matter before, because
>> the only supported format had no padding.
>
> Allwinner DMA engines support only 1, 2, 4 and sometimes 8 bytes for bus
> width, so if sample is 24 bits in size, we have no other way but to transmit
> padding too.

I understand why we do 4 byte DMA from RAM <=> I2S FIFO; that was not my
question. I'm referring to the actual wire format (FIFO <=> PCM_DIN/DOUT). The
sample is already truncated from 32 bits to 24 bits in the FIFO -- that's what
TXIM and RXOM in FIFO_CTRL control.

If a sample is 24 bits wide, why would we send 32 BCLKs for every LRCK? I would
expect the slot width to match the sample resolution by default. But yet we have
this code in the driver:

unsigned int word_size = params_width(params);
unsigned int slot_width = params_physical_width(params);

I think slot_width should be the same as word_size, and I suggest changing it
before adding 20/24-bit support.

> Best regards,
> Jernej

Regards,
Samuel

>>> default:
>>> dev_err(dai->dev, "Unsupported physical sample width:
> %d\n",
>>>
>>> params_physical_width(params));
>>>
>>> @@ -1063,6 +1066,10 @@ static int sun4i_i2s_dai_probe(struct snd_soc_dai
>>> *dai)>
>>> return 0;
>>>
>>> }
>>>
>>> +#define SUN4I_FORMATS (SNDRV_PCM_FMTBIT_S16_LE | \
>>> + SNDRV_PCM_FMTBIT_S20_LE | \
>>> + SNDRV_PCM_FMTBIT_S24_LE)
>>> +
>>>
>>> static struct snd_soc_dai_driver sun4i_i2s_dai = {
>>>
>>> .probe = sun4i_i2s_dai_probe,
>>> .capture = {
>>>
>>> @@ -1070,14 +1077,14 @@ static struct snd_soc_dai_driver sun4i_i2s_dai = {
>>>
>>> .channels_min = 1,
>>> .channels_max = 8,
>>> .rates = SNDRV_PCM_RATE_8000_192000,
>>>
>>> - .formats = SNDRV_PCM_FMTBIT_S16_LE,
>>> + .formats = SUN4I_FORMATS,
>>>
>>> },
>>> .playback = {
>>>
>>> .stream_name = "Playback",
>>> .channels_min = 1,
>>> .channels_max = 8,
>>> .rates = SNDRV_PCM_RATE_8000_192000,
>>>
>>> - .formats = SNDRV_PCM_FMTBIT_S16_LE,
>>> + .formats = SUN4I_FORMATS,
>>>
>>> },
>>> .ops = &sun4i_i2s_dai_ops,
>>> .symmetric_rates = 1,