linux-kernel - [PATCH] [v2] i3c: fix big-endian FIFO transfers

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Message-Id: <20250924201837.3691486-1-arnd@kernel.org>
Date: Wed, 24 Sep 2025 22:18:33 +0200
From: Arnd Bergmann <arnd@...nel.org>
To: Alexandre Belloni <alexandre.belloni@...tlin.com>,
	Jorge Marques <jorge.marques@...log.com>,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	Frank Li <Frank.Li@....com>
Cc: Arnd Bergmann <arnd@...db.de>,
	Manikanta Guntupalli <manikanta.guntupalli@....com>,
	linux-i3c@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] [v2] i3c: fix big-endian FIFO transfers

From: Arnd Bergmann <arnd@...db.de>

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: 733b439375b4 ("i3c: master: Add inline i3c_readl_fifo() and i3c_writel_fifo()")
Cc: Manikanta Guntupalli <manikanta.guntupalli@....com>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
This was a recent regression, the version in 6.16 still works,
but 6.17-rc is broken.

v2 changes:
 - add code comments
 - write correct data buffer
---
 drivers/i3c/internals.h | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/i3c/internals.h b/drivers/i3c/internals.h
index 0d857cc68cc5..79ceaa5f5afd 100644
--- a/drivers/i3c/internals.h
+++ b/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);
 	}
 }
-- 
2.39.5