mtd: spear_smi: Fix Write Burst mode

Any write with either dd or flashcp to a device driven by the
spear_smi.c driver will pass through the spear_smi_cpy_toio()
function. This function will get called for chunks of up to 256 bytes.
If the amount of data is smaller, we may have a problem if the data
length is not 4-byte aligned. In this situation, the kernel panics
during the memcpy:

    # dd if=/dev/urandom bs=1001 count=1 of=/dev/mtd6
    spear_smi_cpy_toio [620] dest c9070000, src c7be8800, len 256
    spear_smi_cpy_toio [620] dest c9070100, src c7be8900, len 256
    spear_smi_cpy_toio [620] dest c9070200, src c7be8a00, len 256
    spear_smi_cpy_toio [620] dest c9070300, src c7be8b00, len 233
    Unhandled fault: external abort on non-linefetch (0x808) at 0xc90703e8
    [...]
    PC is at memcpy+0xcc/0x330

The above error occurs because the implementation of memcpy_toio()
tries to optimize the number of I/O by writing 4 bytes at a time as
much as possible, until there are less than 4 bytes left and then
switches to word or byte writes.

Unfortunately, the specification states about the Write Burst mode:

        "the next AHB Write request should point to the next
	incremented address and should have the same size (byte,
	half-word or word)"

This means ARM architecture implementation of memcpy_toio() cannot
reliably be used blindly here. Workaround this situation by update the
write path to stick to byte access when the burst length is not
multiple of 4.

Fixes: f18dbbb1bfe0 ("mtd: ST SPEAr: Add SMI driver for serial NOR flash")
Cc: Russell King <linux@armlinux.org.uk>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: stable@vger.kernel.org
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Russell King <rmk+kernel@armlinux.org.uk>

drivers/mtd/devices/spear_smi.c

@@ -592,6 +592,26 @@ static int spear_mtd_read(struct mtd_info *mtd, loff_t from, size_t len,
 	return 0;
 }
 
+/*
+ * The purpose of this function is to ensure a memcpy_toio() with byte writes
+ * only. Its structure is inspired from the ARM implementation of _memcpy_toio()
+ * which also does single byte writes but cannot be used here as this is just an
+ * implementation detail and not part of the API. Not mentioning the comment
+ * stating that _memcpy_toio() should be optimized.
+ */
+static void spear_smi_memcpy_toio_b(volatile void __iomem *dest,
+				    const void *src, size_t len)
+{
+	const unsigned char *from = src;
+
+	while (len) {
+		len--;
+		writeb(*from, dest);
+		from++;
+		dest++;
+	}
+}
+
 static inline int spear_smi_cpy_toio(struct spear_smi *dev, u32 bank,
 		void __iomem *dest, const void *src, size_t len)
 {
@@ -614,7 +634,23 @@ static inline int spear_smi_cpy_toio(struct spear_smi *dev, u32 bank,
 	ctrlreg1 = readl(dev->io_base + SMI_CR1);
 	writel((ctrlreg1 | WB_MODE) & ~SW_MODE, dev->io_base + SMI_CR1);
 
-	memcpy_toio(dest, src, len);
+	/*
+	 * In Write Burst mode (WB_MODE), the specs states that writes must be:
+	 * - incremental
+	 * - of the same size
+	 * The ARM implementation of memcpy_toio() will optimize the number of
+	 * I/O by using as much 4-byte writes as possible, surrounded by
+	 * 2-byte/1-byte access if:
+	 * - the destination is not 4-byte aligned
+	 * - the length is not a multiple of 4-byte.
+	 * Avoid this alternance of write access size by using our own 'byte
+	 * access' helper if at least one of the two conditions above is true.
+	 */
+	if (IS_ALIGNED(len, sizeof(u32)) &&
+	    IS_ALIGNED((uintptr_t)dest, sizeof(u32)))
+		memcpy_toio(dest, src, len);
+	else
+		spear_smi_memcpy_toio_b(dest, src, len);
 
 	writel(ctrlreg1, dev->io_base + SMI_CR1);