[PATCH v1] spi: qcom-qspi: Correct max DMA length to avoid 64K boundary failure

[Vijaya Krishna Nivarthi]

From: Vijaya Krishna Nivarthi <vijaya.nivarthi@xxxxxxxxxxxxxxxx>

The maximum size for a DMA data descriptor is 64KB-1 because the size
field in HW is 16 bits wide. For this reason, transfers fail at 64KB
and beyond.

Lower max_dma_len to 60KB so larger transfers are split into multiple
DMA blocks and do not hit the failing 64KB boundary. 60KB is chosen as
a safe round number below the 64KB-1 hardware limit while satisfying
alignment requirements.

Tested on x1e80100 (Hamoa) with SPI-NOR flash (/dev/mtd0):

Without patch:
  dd if=/dev/mtd0 of=/tmp/spi_dump.bin bs=32768 count=2  # works
  dd if=/dev/mtd0 of=/tmp/spi_dump.bin bs=65536 count=1  # fails

With patch:
  dd if=/dev/mtd0 of=/tmp/spi_dump.bin bs=65536 count=1  # works

Fixes: b5762d95607e ("spi: spi-qcom-qspi: Add DMA mode support")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Vijaya Krishna Nivarthi <vijaya.nivarthi@xxxxxxxxxxxxxxxx>
---
 drivers/spi/spi-qcom-qspi.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/spi/spi-qcom-qspi.c b/drivers/spi/spi-qcom-qspi.c
index caf55a6f70b3..e9fdefa9d65d 100644
--- a/drivers/spi/spi-qcom-qspi.c
+++ b/drivers/spi/spi-qcom-qspi.c
@@ -775,7 +775,8 @@ static int qcom_qspi_probe(struct platform_device *pdev)
 		return dev_err_probe(dev, ret, "could not set DMA mask\n");
 
 	host->max_speed_hz = 300000000;
-	host->max_dma_len = 65536; /* as per HPG */
+	/* as per HPG, it is 64KB, limit to 60KB to avoid boundary condition failures */
+	host->max_dma_len = 0xf000;
 	host->dma_alignment = QSPI_ALIGN_REQ;
 	host->num_chipselect = QSPI_NUM_CS;
 	host->bus_num = -1;
-- 
2.34.1