[PATCH 1/2] spi: spi-qcom-qspi: Fallback to PIO for xfers that aren't multiples of 4 bytes

[Douglas Anderson <dianders@xxxxxxxxxxxx>]

The Qualcomm QSPI driver appears to require that any reads using DMA
are a mutliple of 4 bytes. If this isn't true then the controller will
clobber any extra bytes in memory following the last word. Let's
detect this and falback to PIO.

This fixes problems reported by slub_debug=FZPUA, which would complain
about "kmalloc Redzone overwritten". One such instance said:

  0xffffff80c29d541a-0xffffff80c29d541b @offset=21530. First byte 0x0 instead of 0xcc
  Allocated in mtd_kmalloc_up_to+0x98/0xac age=36 cpu=3 pid=6658

Tracing through what was happening I saw that, while we often did DMA
tranfers of 0x1000 bytes, sometimes we'd end up doing ones of 0x41a
bytes. Those 0x41a byte transfers were the problem.

NOTE: a future change will enable the SPI "mem ops" to help avoid this
case, but it still seems good to add the extra check in the transfer.

Fixes: b5762d95607e ("spi: spi-qcom-qspi: Add DMA mode support")
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

 drivers/spi/spi-qcom-qspi.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/spi/spi-qcom-qspi.c b/drivers/spi/spi-qcom-qspi.c
index a0ad9802b606..39b4d8a8107a 100644
--- a/drivers/spi/spi-qcom-qspi.c
+++ b/drivers/spi/spi-qcom-qspi.c
@@ -355,10 +355,22 @@ static int qcom_qspi_setup_dma_desc(struct qcom_qspi *ctrl,
 
 	for (i = 0; i < sgt->nents; i++) {
 		dma_ptr_sg = sg_dma_address(sgt->sgl + i);
+		dma_len_sg = sg_dma_len(sgt->sgl + i);
 		if (!IS_ALIGNED(dma_ptr_sg, QSPI_ALIGN_REQ)) {
 			dev_warn_once(ctrl->dev, "dma_address not aligned to %d\n", QSPI_ALIGN_REQ);
 			return -EAGAIN;
 		}
+		/*
+		 * When reading with DMA the controller writes to memory 1 word
+		 * at a time. If the length isn't a multiple of 4 bytes then
+		 * the controller can clobber the things later in memory.
+		 * Fallback to PIO to be safe.
+		 */
+		if (ctrl->xfer.dir == QSPI_READ && (dma_len_sg & 0x03)) {
+			dev_warn_once(ctrl->dev, "fallback to PIO for read of size %#010x\n",
+				      dma_len_sg);
+			return -EAGAIN;
+		}
 	}
 
 	for (i = 0; i < sgt->nents; i++) {
-- 
2.41.0.487.g6d72f3e995-goog