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

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Message-Id: <20250924150303.3601429-1-arnd@kernel.org>
Date: Wed, 24 Sep 2025 17:02:53 +0200
From: Arnd Bergmann <arnd@...nel.org>
To: Alexandre Belloni <alexandre.belloni@...tlin.com>,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	Jorge Marques <jorge.marques@...log.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] 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.

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.
---
 drivers/i3c/internals.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/i3c/internals.h b/drivers/i3c/internals.h
index 0d857cc68cc5..0f8a25cb71e7 100644
--- a/drivers/i3c/internals.h
+++ b/drivers/i3c/internals.h
@@ -38,7 +38,7 @@ 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(addr, &buf, 1);
 	}
 }

@@ -55,7 +55,7 @@ static inline void i3c_readl_fifo(const void __iomem *addr, void *buf,
 	if (nbytes & 3) {
 		u32 tmp;

-		tmp = readl(addr);
+		readsl(addr, &tmp, 1);
 		memcpy(buf + (nbytes & ~3), &tmp, nbytes & 3);
 	}
 }
-- 
2.39.5