linux-kernel - [PATCH] spi: rockchip: Fix PM runtime count on no-op cs

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Message-ID: <1f2b3af4-2b7a-4ac8-ab95-c80120ebf44c@arm.com>
Date: Fri, 6 Dec 2024 19:50:55 +0000
From: Christian Loehle <christian.loehle@....com>
To: Mark Brown <broonie@...nel.org>, heiko@...ech.de,
 luis.dearquer@...rtim.com
Cc: linux-spi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-rockchip@...ts.infradead.org,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [PATCH] spi: rockchip: Fix PM runtime count on no-op cs

The early bail out that caused an out-of-bounds write was removed with
commit 5c018e378f91 ("spi: spi-rockchip: Fix out of bounds array
access")
Unfortunately that caused the PM runtime count to be unbalanced and
underflowed on the first call. To fix that reintroduce a no-op check
by reading the register directly.

Cc: stable@...r.kernel.org
Fixes: 5c018e378f91 ("spi: spi-rockchip: Fix out of bounds array access")
Signed-off-by: Christian Loehle <christian.loehle@....com>
---
Not particularly happy with it, it's just the most straightforward
fix for the issue at hand. I couldn't quite convince myself that
no-op rockchip_spi_set_cs() are 100% ruled out just from spi.c after
the first call either, but maybe someone more familiar with the code
can. I have only seen the issue for the first call / during boot FWIW.
Any more elegant fix suggestions welcome! The other host controller
drivers don't need the read before CS change, so this shouldn't really
either.

 drivers/spi/spi-rockchip.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/spi/spi-rockchip.c b/drivers/spi/spi-rockchip.c
index 864e58167417..1bc012fce7cb 100644
--- a/drivers/spi/spi-rockchip.c
+++ b/drivers/spi/spi-rockchip.c
@@ -241,6 +241,20 @@ static void rockchip_spi_set_cs(struct spi_device *spi, bool enable)
 	struct spi_controller *ctlr = spi->controller;
 	struct rockchip_spi *rs = spi_controller_get_devdata(ctlr);
 	bool cs_asserted = spi->mode & SPI_CS_HIGH ? enable : !enable;
+	bool cs_actual;
+
+	/*
+	 * SPI subsystem tries to avoid no-op calls that would break the PM
+	 * refcount below. It can't however for the first time it is used.
+	 * To detect this case we read it here and bail out early for no-ops.
+	 */
+	if (spi_get_csgpiod(spi, 0))
+		cs_actual = !!(readl_relaxed(rs->regs + ROCKCHIP_SPI_SER) & 1);
+	else
+		cs_actual = !!(readl_relaxed(rs->regs + ROCKCHIP_SPI_SER) &
+			       BIT(spi_get_chipselect(spi, 0)));
+	if (unlikely(cs_actual == cs_asserted))
+		return;
 
 	if (cs_asserted) {
 		/* Keep things powered as long as CS is asserted */
-- 
2.34.1