[PATCH] ALSA: memalloc: Count continuous pages in vmalloc buffer handler

[Takashi Iwai]

This is an enhancement for the SG-style page handling in vmalloc
buffer handler to calculate the continuous pages.
When snd_sgbuf_get_chunk_size() is called for a vmalloc buffer,
currently we return only the size that fits into a single page.
However, this API call is rather supposed for obtaining the continuous
pages and most of vmalloc or noncontig buffers do have lots of
continuous pages indeed.  So, in this patch, the callback now
calculates the possibly continuous pages up to the given size limit.

Note that the end address in the function is calculated from the last
byte, hence it's one byte shorter.  This is because ofs + size can be
above the actual buffer size boundary.

Until now, this feature isn't really used, but it'll become useful in
a later patch that adds the non-contiguous buffer type that shares the
same callback function as vmalloc.

Link: https://lore.kernel.org/r/20210812113818.6479-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 sound/core/memalloc.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/sound/core/memalloc.c b/sound/core/memalloc.c
index 1cea8cb9668f..c7c943c661e6 100644
--- a/sound/core/memalloc.c
+++ b/sound/core/memalloc.c
@@ -290,11 +290,13 @@ static int snd_dma_vmalloc_mmap(struct snd_dma_buffer *dmab,
 	return remap_vmalloc_range(area, dmab->area, 0);
 }
 
+#define get_vmalloc_page_addr(dmab, offset) \
+	page_to_phys(vmalloc_to_page((dmab)->area + (offset)))
+
 static dma_addr_t snd_dma_vmalloc_get_addr(struct snd_dma_buffer *dmab,
 					   size_t offset)
 {
-	return page_to_phys(vmalloc_to_page(dmab->area + offset)) +
-		offset % PAGE_SIZE;
+	return get_vmalloc_page_addr(dmab, offset) + offset % PAGE_SIZE;
 }
 
 static struct page *snd_dma_vmalloc_get_page(struct snd_dma_buffer *dmab,
@@ -307,11 +309,23 @@ static unsigned int
 snd_dma_vmalloc_get_chunk_size(struct snd_dma_buffer *dmab,
 			       unsigned int ofs, unsigned int size)
 {
-	ofs %= PAGE_SIZE;
-	size += ofs;
-	if (size > PAGE_SIZE)
-		size = PAGE_SIZE;
-	return size - ofs;
+	unsigned int start, end;
+	unsigned long addr;
+
+	start = ALIGN_DOWN(ofs, PAGE_SIZE);
+	end = ofs + size - 1; /* the last byte address */
+	/* check page continuity */
+	addr = get_vmalloc_page_addr(dmab, start);
+	for (;;) {
+		start += PAGE_SIZE;
+		if (start > end)
+			break;
+		addr += PAGE_SIZE;
+		if (get_vmalloc_page_addr(dmab, start) != addr)
+			return start - ofs;
+	}
+	/* ok, all on continuous pages */
+	return size;
 }
 
 static const struct snd_malloc_ops snd_dma_vmalloc_ops = {
-- 
2.26.2