[PATCHv2] mtd: spi-nor: Fix Cadence QSPI page fault kernel panic

[thor.thayer at linux.intel.com]

From: Thor Thayer <thor.thayer at linux.intel.com>

The current Cadence QSPI driver caused a kernel panic when loading
a Root Filesystem from QSPI. The problem was caused by reading more
bytes than needed because the QSPI operated on 4 bytes at a time.
<snip>
[    7.947754] spi_nor_read[1048]:from 0x037cad74, len 1 [bfe07fff]
[    7.956247] cqspi_read[910]:offset 0x58502516, buffer=bfe07fff
[    7.956247]
[    7.966046] Unable to handle kernel paging request at virtual
address bfe08002
[    7.973239] pgd = eebfc000
[    7.975931] [bfe08002] *pgd=2fffb811, *pte=00000000, *ppte=00000000
</snip>
Notice above how only 1 byte needed to be read but by reading 4 bytes
into the end of a mapped page, an unrecoverable page fault occurred.

This patch uses a temporary buffer to hold the 4 bytes read and then
copies only the bytes required into the buffer. A min() function is
used to limit the length to prevent buffer overflows.

Request testing of this patch on other platforms. This was tested
on the Intel Arria10 SoCFPGA DevKit.

Signed-off-by: Thor Thayer <thor.thayer at linux.intel.com>
---
v2  Changes to only write dangling bytes at end of transfer since
      previous patch may have multiple dangling byte transfers.
    Remove write patch since no errors reported and write timeout
      needs more investigation.
---
 drivers/mtd/spi-nor/cadence-quadspi.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/mtd/spi-nor/cadence-quadspi.c b/drivers/mtd/spi-nor/cadence-quadspi.c
index 4b8e9183489a..5872f31eaa60 100644
--- a/drivers/mtd/spi-nor/cadence-quadspi.c
+++ b/drivers/mtd/spi-nor/cadence-quadspi.c
@@ -501,7 +501,9 @@ static int cqspi_indirect_read_execute(struct spi_nor *nor, u8 *rxbuf,
 	void __iomem *reg_base = cqspi->iobase;
 	void __iomem *ahb_base = cqspi->ahb_base;
 	unsigned int remaining = n_rx;
+	unsigned int mod_bytes = n_rx % 4;
 	unsigned int bytes_to_read = 0;
+	u8 *rxbuf_end = rxbuf + n_rx;
 	int ret = 0;
 
 	writel(from_addr, reg_base + CQSPI_REG_INDIRECTRDSTARTADDR);
@@ -530,11 +532,24 @@ static int cqspi_indirect_read_execute(struct spi_nor *nor, u8 *rxbuf,
 		}
 
 		while (bytes_to_read != 0) {
+			unsigned int word_remain = round_down(remaining, 4);
+
 			bytes_to_read *= cqspi->fifo_width;
 			bytes_to_read = bytes_to_read > remaining ?
 					remaining : bytes_to_read;
-			ioread32_rep(ahb_base, rxbuf,
-				     DIV_ROUND_UP(bytes_to_read, 4));
+			bytes_to_read = round_down(bytes_to_read, 4);
+			/* Read 4 byte word chunks then single bytes */
+			if (bytes_to_read) {
+				ioread32_rep(ahb_base, rxbuf,
+					     (bytes_to_read / 4));
+			} else if (!word_remain && mod_bytes) {
+				unsigned int temp = ioread32(ahb_base);
+
+				bytes_to_read = mod_bytes;
+				memcpy(rxbuf, &temp, min((unsigned int)
+							 (rxbuf_end - rxbuf),
+							 bytes_to_read));
+			}
 			rxbuf += bytes_to_read;
 			remaining -= bytes_to_read;
 			bytes_to_read = cqspi_get_rd_sram_level(cqspi);
-- 
2.7.4