linux-kernel - Re: [PATCH v4 01/11] ASoC: jz4740-i2s: Handle independent FIFO flush bits

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Date:   Sat, 22 Oct 2022 16:43:07 +0100
From:   Aidan MacDonald <aidanmacdonald.0x0@...il.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     lgirdwood@...il.com, broonie@...nel.org, perex@...ex.cz,
        tiwai@...e.com, linux-mips@...r.kernel.org,
        alsa-devel@...a-project.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 01/11] ASoC: jz4740-i2s: Handle independent FIFO
 flush bits


Paul Cercueil <paul@...pouillou.net> writes:

> Hi Aidan,
>
> Le mer., juil. 20 2022 at 15:43:06 +0100, Aidan MacDonald
> <aidanmacdonald.0x0@...il.com> a écrit :
>> Paul Cercueil <paul@...pouillou.net> writes:
>>
>> According to the JZ4740 programming manual JZ_AIC_CTRL_FLUSH flushes
>> both FIFOs, so it's not equivalent JZ4760_AIC_CTRL_TFLUSH. I don't
>> think it's a good idea to confuse the two, or we'd need comments to
>> explain why JZ4740 uses TFLUSH but not RFLUSH.
>
> "shared_fifo_flush" is pretty much self-explanatory though. It then becomes
> obvious looking at the code that when this flag is set, TFLUSH flushes both
> FIFOs.
>
> If you prefer... you can #define JZ_AIC_CTRL_FLUSH JZ_AIC_CTRL_TFLUSH. I don't
> like the JZ4760 prefix, this is in no way specific to the JZ4760.
>

Makes sense, I'll stick with TFLUSH / RFLUSH only.

>>
>>>>  +
>>>>   #define JZ_AIC_CTRL_OUTPUT_SAMPLE_SIZE_OFFSET 19
>>>>   #define JZ_AIC_CTRL_INPUT_SAMPLE_SIZE_OFFSET  16
>>>>  @@ -90,6 +93,8 @@ enum jz47xx_i2s_version {
>>>>   struct i2s_soc_info {
>>>>   	enum jz47xx_i2s_version version;
>>>>   	struct snd_soc_dai_driver *dai;
>>>>  +
>>>>  +	bool shared_fifo_flush;
>>>>   };
>>>>   struct jz4740_i2s {
>>>>  @@ -124,12 +129,33 @@ static int jz4740_i2s_startup(struct
>>>> snd_pcm_substream
>>>>  *substream,
>>>>   	uint32_t conf, ctrl;
>>>>   	int ret;
>>>>  +	/*
>>>>  +	 * When we can flush FIFOs independently, only flush the
>>>>  +	 * FIFO that is starting up.
>>>>  +	 */
>>>>  +	if (!i2s->soc_info->shared_fifo_flush) {
>>>>  +		ctrl = jz4740_i2s_read(i2s, JZ_REG_AIC_CTRL);
>>>>  +
>>>>  +		if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
>>>>  +			ctrl |= JZ4760_AIC_CTRL_TFLUSH;
>>>>  +		else
>>>>  +			ctrl |= JZ4760_AIC_CTRL_RFLUSH;
>>>>  +
>>>>  +		jz4740_i2s_write(i2s, JZ_REG_AIC_CTRL, ctrl);
>>>>  +	}
>>>  Wouldn't it be simpler to do one single if/else? And hy is one checked
>>> before
>>>  the (snd_soc_dai_active(dai)) check, and the other is checked after?
>> snd_soc_dai_active() is essentially checking if there's an active
>> substream. Eg. if no streams are open and you start playback, then
>> the DAI will be inactive. If you then start capture while playback is
>> running, the DAI is already active.
>> With a shared flush bit we can only flush if there are no other active
>> substreams (because we don't want to disturb the active stream by
>> flushing the FIFO) so it goes after the snd_soc_dai_active() check.
>> When the FIFOs can be separately flushed, flushing can be done before
>> the check because it won't disturb any active substream.
>
> Ok. It makes sense then. Please add some info about this in the commit message,
> because it really wasn't obvious to me.

It wasn't that obvious to me either :)

I've added code comments too since it seems likely to trip people up
if you're only taking a casual glance.

> You should maybe factorize the read-modify-write into its own function. I know
> this gets eventually modified by [03/11], but this [01/11] is a bugfix so it
> will be applied to older kernels, and I'd rather not have duplicated code
> there.
>
> Cheers,
> -Paul

And I've factored out the r-m-w helper as requested.

Regards,
Aidan