Re: (re)use and (re)definition of snd_pcm_hw_params->fifo_size for 'jumpy DMA'

["Péter Ujfalusi" <peter.ujfalusi@linux.intel.com>]

On 24/03/2026 12:51, Péter Ujfalusi wrote:
> The question is: is it enough to only have single value exposed or do we
> need/want add all small variations of this to be exposed?
> I would think that a single dma_fifo_size should be OK and apps should
> prepare for the crawling DMA case.
> 
>> But if we agree on other name like 'burst_size' or 'batch_size', I'll be
>> fine with it, too.
> 
> I think neither of these are right, unless we substitute them with a
> variable which tells the size of the FIFO in hardware that is going to
> be filled.
> max_hw_ptr_burst_size or max_dma_burst_size or max_burst_size - in frames?

something like this in kernel and then we need alsa-lib update and then we can introduce the use in pipewire?

diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
index d3ce75ba938a..3a1872b3e701 100644
--- a/include/uapi/sound/asound.h
+++ b/include/uapi/sound/asound.h
@@ -421,7 +421,8 @@ struct snd_pcm_hw_params {
 	unsigned int rate_den;		/* R: rate denominator */
 	snd_pcm_uframes_t fifo_size;	/* R: chip FIFO size in frames */
 	unsigned char sync[16];		/* R: synchronization ID (perfect sync - one clock source) */
-	unsigned char reserved[48];	/* reserved for future */
+	snd_pcm_uframes_t max_dma_burst_size;	/* R: maximum DMA burst size in frames */
+	unsigned char reserved[48 - sizeof(snd_pcm_uframes_t)];	/* reserved for future */
 };
 
 enum {
diff --git a/sound/core/pcm_compat.c b/sound/core/pcm_compat.c
index e71f393d3b01..462588f3527b 100644
--- a/sound/core/pcm_compat.c
+++ b/sound/core/pcm_compat.c
@@ -64,7 +64,9 @@ struct snd_pcm_hw_params32 {
 	u32 rate_num;
 	u32 rate_den;
 	u32 fifo_size;
-	unsigned char reserved[64];
+	unsigned char sync[16];
+	u32 max_dma_burst_size;
+	unsigned char reserved[44];
 };
 
 struct snd_pcm_sw_params32 {
@@ -247,9 +249,11 @@ static int snd_pcm_ioctl_hw_params_compat(struct snd_pcm_substream *substream,
 	if (!data)
 		return -ENOMEM;
 
-	/* only fifo_size (RO from userspace) is different, so just copy all */
+	/* copy common members and fix up 32-bit uframe fields explicitly */
 	if (copy_from_user(data, data32, sizeof(*data32)))
 		return -EFAULT;
+	data->fifo_size = data32->fifo_size;
+	data->max_dma_burst_size = data32->max_dma_burst_size;
 
 	if (refine) {
 		err = snd_pcm_hw_refine(substream, data);
@@ -262,7 +266,8 @@ static int snd_pcm_ioctl_hw_params_compat(struct snd_pcm_substream *substream,
 	if (err < 0)
 		return err;
 	if (copy_to_user(data32, data, sizeof(*data32)) ||
-	    put_user(data->fifo_size, &data32->fifo_size))
+	    put_user(data->fifo_size, &data32->fifo_size) ||
+	    put_user(data->max_dma_burst_size, &data32->max_dma_burst_size))
 		return -EFAULT;
 
 	if (! refine) {


-- 
Péter