linux-kernel - Re: Page select register restrictions in regmap core

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Message-ID: <2b3d76b8-11a1-4235-9a62-85d6e7917147@roeck-us.net>
Date: Tue, 18 Jun 2024 20:11:53 -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: Re: Page select register restrictions in regmap core

On 6/18/24 13:46, Mark Brown wrote:
> On Tue, Jun 18, 2024 at 12:33:40PM -0700, Guenter Roeck wrote:
>> On 6/18/24 10:45, Mark Brown wrote:
>>> On Tue, Jun 18, 2024 at 09:14:56AM -0700, Guenter Roeck wrote:
> 
>>>> It turns out that at least some i801 controllers don't work with the
>>>> access mechanism used by regmap, or maybe the spd5118 chips don't support
>>>> I2C_FUNC_SMBUS_I2C_BLOCK. I already found that those chips don't support
>>>> auto-incrementing the register address and actually reset the address on byte
>>>> reads (i.e., subsequent calls to i2c_smbus_read_byte() always return the data
>>>> from the first register). Since regmap doesn't have a means for me to say
>>>> "don't use I2C_FUNC_SMBUS_I2C_BLOCK even if the controller supports it",
>>>> I may have to drop regmap support entirely anyway. That would be annoying,
>>>> but right now I have no idea how to work around that problem.
> 
>>> You can set the use_single_read and use_single_write flags in the config
>>> to ensure registers are accessed one at a time, that restriction is
>>> moderately common.
> 
>> That doesn't help, unfortunately. Thinking about it, that is not really
>> surprising. The failing write is to the page register, and that was
>> a single write anyway.
> 
> Oh, that's interesting - I'm kind of surprised the wire protocols differ
> but it's been a while since I looked.  We should probably add this to

Magic solved: As it turns out, recent i801 controllers have a write protect
bit which can be set by the BIOS and prevents writes in the 0x50..0x57 i2c
address range. Apparently this is what happened here. Side effect is that
it is impossible to read the spd5118 eeprom from the operating system
if the write protect bit is set since doing that requires a write to set
the page register.

Sorry for the noise.

Thanks,
Guenter