linux-kernel - Re: [RFC 0/7] iio: inv_mpu6050: Support i2c master and external readings

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Date:	Tue, 3 May 2016 14:21:40 +0300
From:	Crestez Dan Leonard <leonard.crestez@...el.com>
To:	Mark Brown <broonie@...nel.org>,
	Jonathan Cameron <jic23@...nel.org>
Cc:	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
	Hartmut Knaack <knaack.h@....de>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Peter Meerwald-Stadler <pmeerw@...erw.net>,
	Daniel Baluta <daniel.baluta@...el.com>,
	Ge Gao <GGao@...ensense.com>, Peter Rosin <peda@...ntia.se>
Subject: Re: [RFC 0/7] iio: inv_mpu6050: Support i2c master and external
 readings

On 05/02/2016 06:23 PM, Mark Brown wrote:
> On Sun, May 01, 2016 at 06:04:08PM +0100, Jonathan Cameron wrote:
> 
>> If you were to break these registers up into regmap fields it might solve 
>> this..  Regmap writes always go through whatever - whether they match the
>> existing state of the cache or not.  If fields are involved the write will get
>> built up from whatever field you change and whatever the cache has for other
>> elements.  I guess it only works if they volatile bits are contiguous though.
>> Maybe hand rolling it is cleaner here.
> 
>> Mark, any clever thoughts on this?
> 
> I don't have enough context here to be sure what the problem you're
> trying to solve is, sorry.
> 
This is worth explaining:

I have a device which has several registers with bits that are a mix of
"cacheable" and "volatile". For example for register SLV4_CTRL:

- Bit 7 (I2C_SLV4_EN) triggers a transaction with slave 4 when a "1" is
written. The bit is cleared when the transaction is done.
- Bits 0-4 (I2C_MST_DLY) configures the reduced access rate of I2C
slaves relative to the device sample rate. This applies to slaves 0-3 as
well.

If I2C_MST_DLY was a separate register it could be easily cached by
regmap. Because it's part of a volatile register I have to add a
private_data field caching the value and always write it when triggering
a SLV4 transfer.

Jonathan was wondering if regmap can still be used somehow instead of
custom caching.

-- 
Regards,
Leonard