spi: Fix scatterlist elements size in spi_map_buf

When SPI transfers can be offloaded using DMA, the SPI core need to
build a scatterlist to make sure that the buffer to be transferred is
dma-able.

This patch fixes the scatterlist entry size computation in the case
where the maximum acceptable scatterlist entry supported by the DMA
controller is less than PAGE_SIZE, when the buffer is vmalloced.

For each entry, the actual size is given by the minimum between the
desc_len (which is the max buffer size supported by the DMA controller)
and the remaining buffer length until we cross a page boundary.

Fixes: 65598c13fd ("spi: Fix per-page mapping of unaligned vmalloc-ed buffer")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org

drivers/spi/spi.c

@@ -779,8 +779,14 @@ static int spi_map_buf(struct spi_controller *ctlr, struct device *dev,
 	for (i = 0; i < sgs; i++) {
 
 		if (vmalloced_buf || kmap_buf) {
-			min = min_t(size_t,
-				    len, desc_len - offset_in_page(buf));
+			/*
+			 * Next scatterlist entry size is the minimum between
+			 * the desc_len and the remaining buffer length that
+			 * fits in a page.
+			 */
+			min = min_t(size_t, desc_len,
+				    min_t(size_t, len,
+					  PAGE_SIZE - offset_in_page(buf)));
 			if (vmalloced_buf)
 				vm_page = vmalloc_to_page(buf);
 			else