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

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Date: Tue, 18 Jun 2024 14:23:07 -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

spd5118 devices are ... weird. For example, they don't auto-increment
addresses on a read operation, meaning regmap_i2c_smbus_i2c_read_reg16()
doesn't work at all because i2c_smbus_read_byte() always reads from
address 0.

> the quirking in regmap-i2c.c, have it select one the 8 bit only smbus
> versions for devices that need register at a time operation.  I'd not be
> surprised if other devices have issues, and anyway if it makes a
> difference to the wire protocol we should try to select something as
> close as possible to what we're actually doing.
> 
> Something like the below perhaps (this probably needs to be converted to
> a match table type thing at this point, there's the SMBUS_WORD_DATA case
> too):
> 
> diff --git a/drivers/base/regmap/regmap-i2c.c b/drivers/base/regmap/regmap-i2c.c
> index a905e955bbfc..499fcec00f2d 100644
> --- a/drivers/base/regmap/regmap-i2c.c
> +++ b/drivers/base/regmap/regmap-i2c.c
> @@ -313,6 +313,11 @@ static const struct regmap_bus *regmap_get_i2c_bus(struct i2c_client *i2c,
>   
>   	if (i2c_check_functionality(i2c->adapter, I2C_FUNC_I2C))
>   		bus = &regmap_i2c;
> +	else if (config->val_bits == 8 && config->reg_bits == 8 &&
> +		 config->use_single_read && config->use_single_write &&
> +		 i2c_check_functionality(i2c->adapter,
> +					 I2C_FUNC_SMBUS_BYTE_DATA))
> +		bus = &regmap_smbus_byte;

That might be an option. Give me more time, though - as it turns out,
I2C_FUNC_SMBUS_BYTE_DATA doesn't work for writes either on the affected
system. Something else is going on.

Thanks,
Guenter