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

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Date:	Thu, 06 Jun 2013 12:34:57 +0100
From:	Nick Dyer <nick.dyer@...ev.co.uk>
To:	Mark Brown <broonie@...nel.org>
CC:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	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

Mark Brown wrote:
> On Thu, Jun 06, 2013 at 12:00:54PM +0100, Nick Dyer wrote:
>> Mark Brown wrote:
> 
>>> The retries can just be done further up the stack?  All regmap is doing
>>> with I/O errors is punting them straight back up to the caller so the
>>> caller can retry just as well using regmap as it can using the raw I/O
>>> protocol.
> 
>> It would have to be put into users of the debugfs interface as well.
>> There's quite tight timing required to make it work properly (see patch
>> [40/53]).
> 
> This is yet another reason for implementing the protocol properly
> instead of trying to bodge around the kernel.  It really seems like
> the biggest problem here is the decision to try to bodge the entire
> thing into userspace with no kernel support.

With the interface I am proposing it is handled properly, in the kernel driver.

>From an Atmel perspective, Linux is just another platform and we want to
use our existing investment in tools and documentation to manage & debug
chips embedded in Linux based devices. So providing a bridge using a
relatively simple API between the tools and the kernel driver is the
correct decision. I can't provide a 3D graph of live touch data in the
kernel driver, for instance.

>>> Without seeing the address thing it's hard to comment.
> 
>> Patch [36/53]. If the T5 message processor is from address 100-110, you can
>> do a read of 50 bytes starting at address 100, and it will return 10
>> messages, but anything in regmap that tries to do bounds checking would get
>> confused, I think.
> 
> That's just not going to be supported, sorry.  You can implement custom
> locks and access the device directly where you need to do stuff like
> that while still using regmap for actual registers though.

OK, fair enough.

>> Also, we would like to implement address pointer caching. maXTouch allows
>> us to skip the address part of the i2c transaction if the address pointer
>> in the chip hasn't changed. This speeds up interrupt handler slightly. But
>> it requires extra housekeeping at a low level to remember what the address
>> pointer was on the previous transaction to know whether to send it or not.
> 
> That sounded like what you were talking about, it's pretty common and is
> sane enough for reads.

The address pointer is shared between reads and writes on maXTouch, but I
guess that's not a huge problem.
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