linux-kernel - Page select register restrictions in regmap core

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Message-ID: <e3e11724-794d-423e-9326-ffe8eed5119c@roeck-us.net>
Date: Mon, 17 Jun 2024 07:20:28 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Mark Brown <broonie@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, Armin Wolf <W_Armin@....de>
Subject: Page select register restrictions in regmap core

Hi,

I have been trying to use the regmap page select mechanism to implement
page access in the upcoming spd5118 driver [1]. Unfortunately, that fails
with the following error.

spd5118 0-0050: Range 0: selector for 1 in window
spd5118 0-0050: error -EINVAL: regmap init failed

The page select register in spd5118 devices is outside the paged area
in an area which is otherwise (i.e., for other registers in that area)
only accessible from page 0. The regmap range configuration looks as
follows.

static const struct regmap_range_cfg regmap_range_cfg[] = {
	/*
	 * volatile registers, only accessible from page 0 except for
	 * the page select register
	 */
        {
        .selector_reg     = SPD5118_REG_I2C_LEGACY_MODE,	// 0x0b
        .selector_mask    = SPD5118_LEGACY_PAGE_MASK,		// 0x07
        .selector_shift   = 0,
        .window_start     = 0,
        .window_len       = SPD5118_PAGE_SIZE,			// 128
        .range_min        = 0,
        .range_max        = SPD5118_PAGE_SIZE - 1,		// 127
        },
	/* non-volatile data, pages 0..7 */
        {
        .selector_reg     = SPD5118_REG_I2C_LEGACY_MODE,	// 0x0b
        .selector_mask    = SPD5118_LEGACY_PAGE_MASK,		// 0x07
        .selector_shift   = 0,
        .window_start     = SPD5118_PAGE_SIZE,			// 128
        .window_len       = SPD5118_PAGE_SIZE,			// 128
        .range_min        = SPD5118_PAGE_SIZE,			// 128
        .range_max        = 9 * SPD5118_PAGE_SIZE - 1,		// 0x47f
        },
};

This works just fine if I comment out the select register validation in
the regmap core (the one which generates the error). Is there a reason
for having this restriction, or would it be possible to drop it ?
If dropping it is not possible, could we possibly add some flag to the
configuration data, indicating that this "violation" is on purpose ?
Alternatively, do you see some other means to describe the ranges which
would not violate the restrictions ?

Thanks,
Guenter

---
[1] https://patchwork.kernel.org/project/linux-hwmon/patch/20240610144103.1970359-3-linux@roeck-us.net/