i3c: fix big-endian FIFO transfers

Short MMIO transfers that are not a multiple of four bytes in size need
a special case for the final bytes, however the existing implementation
is not endian-safe and introduces an incorrect byteswap on big-endian
kernels.

This usually does not cause problems because most systems are
little-endian and most transfers are multiple of four bytes long, but
still needs to be fixed to avoid the extra byteswap.

Change the special case for both i3c_writel_fifo() and i3c_readl_fifo()
to use non-byteswapping writesl() and readsl() with a single element
instead of the byteswapping writel()/readl() that are meant for individual
MMIO registers. As data is copied between a FIFO and a memory buffer,
the writesl()/readsl() loops are typically based on __raw_readl()/
__raw_writel(), resulting in the order of bytes in the FIFO to match
the order in the buffer, regardless of the CPU endianess.

The earlier versions in the dw-i3c and i3c-master-cdns had a correct
implementation, but the generic version that was recently added broke it.

Fixes: 733b439375 ("i3c: master: Add inline i3c_readl_fifo() and i3c_writel_fifo()")
Cc: Manikanta Guntupalli <manikanta.guntupalli@amd.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jorge Marques <jorge.marques@analog.com>
Link: https://lore.kernel.org/r/20250924201837.3691486-1-arnd@kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/i3c/internals.h

@@ -38,7 +38,11 @@ static inline void i3c_writel_fifo(void __iomem *addr, const void *buf,
 		u32 tmp = 0;
 
 		memcpy(&tmp, buf + (nbytes & ~3), nbytes & 3);
-		writel(tmp, addr);
+		/*
+		 * writesl() instead of writel() to keep FIFO
+		 * byteorder on big-endian targets
+		 */
+		writesl(addr, &tmp, 1);
 	}
 }
 
@@ -55,7 +59,11 @@ static inline void i3c_readl_fifo(const void __iomem *addr, void *buf,
 	if (nbytes & 3) {
 		u32 tmp;
 
-		tmp = readl(addr);
+		/*
+		 * readsl() instead of readl() to keep FIFO
+		 * byteorder on big-endian targets
+		 */
+		readsl(addr, &tmp, 1);
 		memcpy(buf + (nbytes & ~3), &tmp, nbytes & 3);
 	}
 }