linux-kernel - Re: [PATCH] iio: max5487: Add support for Maxim digital potentiometers

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Message-ID: <CAEnQRZAXPRa+8M7ga1T9pOPUb6pmbGScHOLD1dqd9Dvpg7GsNg@mail.gmail.com>
Date:	Fri, 1 Apr 2016 11:33:03 +0300
From:	Daniel Baluta <daniel.baluta@...el.com>
To:	Peter Rosin <peda@...ntia.se>
Cc:	Cristina Moraru <cristina.moraru09@...il.com>,
	"jic23@...nel.org" <jic23@...nel.org>,
	"knaack.h@....de" <knaack.h@....de>,
	"lars@...afoo.de" <lars@...afoo.de>,
	"pmeerw@...erw.net" <pmeerw@...erw.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
	"daniel.baluta@...el.com" <daniel.baluta@...el.com>,
	"octavian.purdila@...el.com" <octavian.purdila@...el.com>
Subject: Re: [PATCH] iio: max5487: Add support for Maxim digital potentiometers

On Fri, Mar 25, 2016 at 12:20 PM, Peter Rosin <peda@...ntia.se> wrote:
> Hi again,
>
> Cristina Moraru wrote:
>> Add implementation for Maxim MAX5487, MAX5488, MAX5489
>> digital potentiometers.
>>
>> Signed-off-by: Cristina Moraru <cristina.moraru09@...il.com>
>> CC: Daniel Baluta <daniel.baluta@...el.com>
>
> Some more comments, the mcp4531 chips have n**2 + 1 positions,
> therefore .max_pos in that driver isn't the number of wiper positions, it's
> the actual maximum value. So, in this driver, the corrent number for
> .max_pos would be 255, otherwise the reported scale is wrong (and then
> you also need to adjust the EINVAL check in max5487_write_raw to use >
> instead of >=).
>
> Further comparison with the mcp4531 driver reveals that this driver does
> not support IIO_CHAN_INFO_RAW in max5487_read_raw. I assume the SPI
> interface does not support reading back the current value?

Yes. The registers are write only. We could have used the caching
facility of regmap with default values.

The problem is when using the non volatile (NV) mem there is no way to know
the previous wiper position.

Daniel.