[PATCH] spi: fix zero-length scatterlist entries for vmalloc buffers

[å¼ æ˜¥å‘ <zcf1810@163.com>]

spi_map_buf_attrs() sizes the sg table with

	DIV_ROUND_UP(len + offset_in_page(buf), desc_len)

but fills the entries with

	min(desc_len, min(len, PAGE_SIZE - offset_in_page(buf)))

Those two expressions only agree when a page boundary coincides with a
chunk boundary.  On 64K pages with a controller whose max_seg_size is
smaller than PAGE_SIZE, a buffer that is not page aligned produces a
table that is one or more entries too large, and the trailing entries are
filled with length 0.

dma_map_sgtable() maps every entry up to sgt->orig_nents, so such an
entry reaches dma_direct_map_sg() -> dma_direct_map_page(size = 0).
dma_capable() computes end = addr + size - 1, i.e. addr - 1 for a zero
sized mapping, which is false for any buffer above the device DMA mask,
and swiotlb_map() then calls swiotlb_tbl_map_single(size = 0) ->
swiotlb_do_find_slots(), where

	BUG_ON(!nslots);

fires because nr_slots(0) is 0.  On 4K pages desc_len always equals
PAGE_SIZE and the page boundary absorbs the offset, which is why this has
only been observed after switching to 64K pages.

Fix the entry count to match the way the loop consumes the buffer, and skip
zero length transfers entirely.

Signed-off-by: 张春发 <zcf1810@163.com>
---
Validated against the fill loop with 20000 random combinations of
(len, offset_in_page, PAGE_SIZE in {4K,16K,64K}, max_seg_size): no
zero-length entries are produced and the segment lengths always sum to
len.  Not boot-tested on hardware; the reporter hits the BUG on a 6.1.158
arm64 board with 64K pages.

 drivers/spi/spi.c | 29 +++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -1339,9 +1339,34 @@ static int spi_map_buf_attrs(struct spi_controller *ctlr, struct device *dev,
 	size_t min;
 	int i, ret;
 
+	if (!len) {
+		/*
+		 * Zero-length transfers need no DMA mapping.  Leave an empty
+		 * table behind so spi_unmap_buf_attrs() sees orig_nents == 0.
+		 */
+		memset(sgt, 0, sizeof(*sgt));
+		return 0;
+	}
+
 	if (vmalloced_buf || kmap_buf) {
-		desc_len = min_t(unsigned long, max_seg_size, PAGE_SIZE);
-		sgs = DIV_ROUND_UP(len + offset_in_page(buf), desc_len);
+		size_t off = offset_in_page(buf);
+		size_t first = min_t(size_t, len, PAGE_SIZE - off);
+		size_t rest;
+
+		desc_len = min_t(size_t, max_seg_size, PAGE_SIZE);
+		/*
+		 * The buffer is first split at page boundaries, then every
+		 * piece is split into desc_len chunks.  Sizing the table with
+		 * DIV_ROUND_UP(len + off, desc_len) does not model that and
+		 * leaves zero-length entries whenever a page boundary does
+		 * not coincide with a chunk boundary (which trips
+		 * BUG_ON(!nslots) in swiotlb on 64K pages).
+		 */
+		sgs = DIV_ROUND_UP(first, desc_len);
+		rest = len - first;
+		sgs += (rest / PAGE_SIZE) * DIV_ROUND_UP(PAGE_SIZE, desc_len);
+		if (rest % PAGE_SIZE)
+			sgs += DIV_ROUND_UP(rest % PAGE_SIZE, desc_len);
 	} else if (virt_addr_valid(buf)) {
 		desc_len = min_t(size_t, max_seg_size, ctlr->max_dma_len);
 		sgs = DIV_ROUND_UP(len, desc_len);