linux-kernel - Re: BUG: gpio: pca953x: 24 bit expanders broken since v5.2-rc1

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Date:   Mon, 3 Jun 2019 10:08:08 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Nandor Han <nandor.han@...sala.com>,
        Mark Brown <broonie@...nel.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Tony Lindgren <tony@...mide.com>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>, kernel@...a-handheld.com,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: BUG: gpio: pca953x: 24 bit expanders broken since v5.2-rc1


> Am 01.06.2019 um 23:57 schrieb Linus Walleij <linus.walleij@...aro.org>:
> 
> On Fri, May 31, 2019 at 7:06 PM H. Nikolaus Schaller <hns@...delico.com> wrote:
> 
>> Now, this was not a (visible) problem until patch
>> 
>>        8b9f9d4dc511 regmap: verify if register is writeable before writing operations
>> 
>> enforces to check the register number before invoking the
>> callback pca953x_writeable_register(). pca953x_writeable_register()
>> seems to know about REG_ADDR_AI (through reg & REG_ADDR_MASK) and
>> accepts 0x88 as a valid register number.
>> 
>> After the regmap patch the register is checked against
>> pca953x_i2c_regmap.max_register before applying REG_ADDR_MASK
>> and 0x88 is obviously beyond, explaining the symptom.
> 
> Can we simply bump the .max_register in
> pca953x_i2c_regmap to 0xff for a quick fix with a comment
> FIXME to figure it out the right way?

Yes, that might be a quick workaround closer to the correct code location
in the driver.

There seem not to be many regmap accesses with randomly toggled REG_ADDR_AI
bit and therefore risk of seeing two different cache entries where they would
assume the same is low (and not higher than before we fix anything).

I'll give it a try asap.

BR,
Nikolaus