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

[Takashi Iwai <tiwai@suse.de>]

On Thu, 16 Jan 2020 10:50:33 +0100,
Keyon Jie wrote:
> 
> 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?
> ...
> }

Oh, you're right, and I completely misread the patch.

Now I took a coffee and can tell you the story behind the scene.

I believe the current code is intentionally limiting the size to the
preallocated size.  This limitation was brought for not trying to
allocate a larger buffer when the buffer has been preallocated.  In
the past, most hardware allocated the continuous pages for a buffer
and the allocation of a large buffer fails quite likely.  This was the
reason of the buffer preallocation.  So, the driver wanted to tell the
user-space the limit.  If user needs to have an extra large buffer,
they are supposed to fiddle with prealloc procfs (either setting zero
to clear the preallocation or setting a large enough buffer
beforehand).

For SG-buffers, though, limitation makes less sense than continuous
pages.  e.g. a patch below removes the limitation for SG-buffers.
But changing this would definitely cause the behavior difference, and
I don't know whether it's a reasonable move -- I'm afraid that apps
would start hogging too much memory if the limitation is gone.


thanks,

Takashi

---
diff --git a/sound/core/pcm_memory.c b/sound/core/pcm_memory.c
index d4702cc1d376..6a6c3469bbcd 100644
--- a/sound/core/pcm_memory.c
+++ b/sound/core/pcm_memory.c
@@ -96,6 +96,29 @@ void snd_pcm_lib_preallocate_free_for_all(struct snd_pcm *pcm)
 }
 EXPORT_SYMBOL(snd_pcm_lib_preallocate_free_for_all);
 
+/* set up substream->buffer_bytes_max, which is used in hw_constraint */
+static void set_buffer_bytes_max(struct snd_pcm_substream *substream,
+				 size_t size)
+{
+	substream->buffer_bytes_max = UINT_MAX;
+
+	if (!size)
+		return; /* no preallocation */
+
+	/* for SG-buffers, no limitation is needed */
+	switch (substream->dma_buffer.dev.type) {
+#ifdef CONFIG_SND_DMA_SGBUF
+	case SNDRV_DMA_TYPE_DEV_SG:
+	case SNDRV_DMA_TYPE_DEV_UC_SG:
+#endif
+	case SNDRV_DMA_TYPE_VMALLOC:
+		return;
+	}
+
+	/* for continuous buffers, limit to the preallocated size */
+	substream->buffer_bytes_max = size;
+}
+
 #ifdef CONFIG_SND_VERBOSE_PROCFS
 /*
  * read callback for prealloc proc file
@@ -156,10 +179,8 @@ static void snd_pcm_lib_preallocate_proc_write(struct snd_info_entry *entry,
 				buffer->error = -ENOMEM;
 				return;
 			}
-			substream->buffer_bytes_max = size;
-		} else {
-			substream->buffer_bytes_max = UINT_MAX;
 		}
+		set_buffer_bytes_max(substream, size);
 		if (substream->dma_buffer.area)
 			snd_dma_free_pages(&substream->dma_buffer);
 		substream->dma_buffer = new_dmab;
@@ -206,10 +227,8 @@ static void preallocate_pages(struct snd_pcm_substream *substream,
 
 	if (size > 0 && preallocate_dma && substream->number < maximum_substreams)
 		preallocate_pcm_pages(substream, size);
-
-	if (substream->dma_buffer.bytes > 0)
-		substream->buffer_bytes_max = substream->dma_buffer.bytes;
 	substream->dma_max = max;
+	set_buffer_bytes_max(substream, substream->dma_buffer.bytes);
 	if (max > 0)
 		preallocate_info_init(substream);
 	if (managed)
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