linux-kernel - Re: [PATCH 10/53] Input: atmel_mxt_ts - Add memory access interface via sysfs

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Message-ID: <51AFA02B.3000604@itdev.co.uk>
Date:	Wed, 05 Jun 2013 21:31:39 +0100
From:	Nick Dyer <nick.dyer@...ev.co.uk>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
CC:	Daniel Kurtz <djkurtz@...omium.org>,
	Henrik Rydberg <rydberg@...omail.se>,
	Joonyoung Shim <jy0922.shim@...sung.com>,
	Alan.Bowens@...el.com, linux-input@...r.kernel.org,
	linux-kernel@...r.kernel.org, pmeerw@...erw.net,
	bleung@...omium.org, olofj@...omium.org
Subject: Re: [PATCH 10/53] Input: atmel_mxt_ts - Add memory access interface
 via sysfs

Dmitry Torokhov wrote:
>> We have made a deliberate choice to implement this via sysfs rather than
>> debugfs since it needs to work on devices that don't have debugfs enabled.
> 
> Then they will have to enable it. Re-implementing something because
> someone might not enable needed subsystem is not a good idea. Let's
> say somebody disabled I2C - will you write your own implementation because
> of that? Of course not, you just say that for given functionality it
> is a prerequisite.

debugfs is a debug filesystem. This interface is useful for purposes which
are not debug. I have to be pragmatic: I don't see debugfs enabled on most
shipping Android devices, and however much I tell them to enable debugfs
doesn't seem to hold much weight.

It's partly path dependence - it was implemented like this because regmap
wasn't in mainline at the point when I wrote it. Having a dependency on
regmap would now be a API break complicating support of customers using
older kernels than mainline. I would also have to update a bunch of
software and documentation and people to know about the two different APIs.
The existing implementation already appears in shipping devices, so it is
well tested.

>> In addition, there are some quirks about the way in which we have to
>> read/write registers which means regmap isn't a good fit.
> 
> Could you please elaborate more on this?

- the mxt chip caches the I2C read pointer, so you can get a performance
optimisation by not sending the address on every read/write (I haven't
implemented this yet but plan to)
- the address pointer can wrap around when you read the T5 message
processor, which would confuse regmap
- we require I2C retries in some cases due to way the chip handles sleep states
- I can't see how to map the object protocol (used on mxt chips) into the
way regmap treats register ranges

I can look into porting on top of regmap. But it seems a pity to pepper
regmap with atmel_mxt_ts quirks just to save on three small functions in
the driver.

Thanks for your time to look at this.

N
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