linux-kernel - [PATCH] gpio: pca953x: hack to fix 24 bit gpio expanders

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Date:   Tue,  4 Jun 2019 14:35:58 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Cc:     linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
        letux-kernel@...nphoenux.org, kernel@...a-handheld.com,
        "H. Nikolaus Schaller" <hns@...delico.com>
Subject: [PATCH] gpio: pca953x: hack to fix 24 bit gpio expanders

24 bit expanders use REG_ADDR_AI in combination with register addressing. This
conflicts with regmap which takes this bit as part of the register number,
i.e. a second cache entry is defined for accessed with REG_ADDR_AI being
set although on the chip it is the same register as with REG_ADDR_AI being
cleared.

The problem was introduced by

	commit b32cecb46bdc ("gpio: pca953x: Extract the register address mangling to single function")

but only became visible by

	commit 8b9f9d4dc511 ("regmap: verify if register is writeable before writing operations")

because before, the regmap size was effectively ignored and
pca953x_writeable_register() did know to ignore REG_ADDR_AI. Still, there
were two separate cache entries created.

Since the use of REG_ADDR_AI seems to be static we can work around this
issue by simply increasing the size of the regmap to cover the "virtual"
registers with REG_ADDR_AI being set. This only means that half of the
regmap buffer will be unused.

Reported-by: H. Nikolaus Schaller <hns@...delico.com>
Suggested-by: Linus Walleij <linus.walleij@...aro.org>
Signed-off-by: H. Nikolaus Schaller <hns@...delico.com>
---
 drivers/gpio/gpio-pca953x.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpio/gpio-pca953x.c b/drivers/gpio/gpio-pca953x.c
index b7ef33f63392..2b27d18410aa 100644
--- a/drivers/gpio/gpio-pca953x.c
+++ b/drivers/gpio/gpio-pca953x.c
@@ -308,7 +308,8 @@ static const struct regmap_config pca953x_i2c_regmap = {
 	.volatile_reg = pca953x_volatile_register,

 	.cache_type = REGCACHE_RBTREE,
-	.max_register = 0x7f,
+	/* REVISIT: should be 0x7f but some 24 bit chips use REG_ADDR_AI */
+	.max_register = 0xff,
 };

 static u8 pca953x_recalc_addr(struct pca953x_chip *chip, int reg, int off,
-- 
2.19.1