[alsa-devel] [PATCH] ALSA: pcm: fix buffer_bytes max constrained by preallocated bytes issue

[Keyon Jie]

On Thu, 2020-01-16 at 08:15 +0100, Takashi Iwai wrote:
> On Thu, 16 Jan 2020 05:53:18 +0100,
> Keyon Jie wrote:
> > With today's code, we preallocate DMA buffer for substreams at
> > pcm_new()
> > stage, and the substream->buffer_bytes_max and substream->dma_max
> > will
> > save as the actually preallocated buffer size and maximum size that
> > the
> > dma buffer can be expanded by at hw_params() state,
> > correspondingly.
> 
> No, it's other way round: the former, buffer_bytes_max, is the max
> size defined by the driver (i.e. passed in snd_pcm_hardware) and the
> latter, dma_max, is the max preallocation size (passed to
> preallocation helper).

Hi Takashi, thanks for your comment.

First of all, have you ever hit issue I mentioned in the commit message
that we can't set buffer_bytes larger than the preallocated dma bytes?

I found this issue in kinds of platforms, not only on SOF/SoC ones, but
also on legacy HDA ones.

Secondly, I am not clear about the design intention of the substream-
>buffer_bytes_max and substream->dma_max, if it is as you commented
above, can you help answer my questions below inline the code?

void snd_pcm_lib_preallocate_pages(struct snd_pcm_substream *substream,
				  int type, struct device *data,
				  size_t size, size_t max)

static void preallocate_pages(struct snd_pcm_substream *substream,
			      int type, struct device *data,
			      size_t size, size_t max, bool managed)
{
...
	if (substream->dma_buffer.bytes > 0)
		substream->buffer_bytes_max = substream-
>dma_buffer.bytes;//Keyon: this is the actual allocated buffer bytes,
what is the intention here and why it is assigned to buffer_bytes_max
which will be used to constrain on the _HW_PARAM_BUFFER_BYTES later?

	substream->dma_max = max; //Keyon: looks here it is where the
*max* param used only if we don't define SND_VERBOSE_PROCFS? what
relationship can we have with the preallocation itself?
...
}


> 
> > At pcm_open() stage, the maximum constraint of
> > HW_PARAM_BUFFER_BYTES is
> > set to substream->buffer_bytes_max and returned to user space as
> > the max
> > interval of the HW_PARAM_BUFFER_BYTES, this will lead to issue that
> > user
> > can't choose any buffer-bytes larger than the preallocated buffer
> > size,
> > and the buffer reallocation will never happen actually.
> > 
> > Here change to use substream->dma_max as the maximum constraint of
> > the
> > HW_PARAM_BUFFER_BYTES and fix the issue mentioned above.
> 
> I don't think the logic in the current code you're changing is wrong.
> If there is any, it must be something else.
> 
> This might be rather the FIXME code found in
> snd_pcm_hw_constraints_complete()?

I just tried removing the FIXME part code and it doesn't help, the rule
snd_pcm_hw_rule_buffer_bytes_max here limit the max of the
SNDRV_PCM_HW_PARAM_BUFFER_BYTES and this will returned to user space
like aplay for the subsequent hw_params(), is this intentional?

int snd_pcm_hw_constraints_complete(struct snd_pcm_substream
*substream)
{
...
	err = snd_pcm_hw_rule_add(runtime, 0,
SNDRV_PCM_HW_PARAM_BUFFER_BYTES, 
				  snd_pcm_hw_rule_buffer_bytes_max,
substream,
				  SNDRV_PCM_HW_PARAM_BUFFER_BYTES, -1);
	if (err < 0)
		return err;

	/* FIXME: remove */
	if (runtime->dma_bytes) {
		err = snd_pcm_hw_constraint_minmax(runtime,
SNDRV_PCM_HW_PARAM_BUFFER_BYTES, 0, runtime->dma_bytes);
		if (err < 0)
			return err;
	}

...
	return 0;
}

Thanks,
~Keyon