linux-kernel - Re: [PATCH 06/20] Input: atmel_mxt_ts - allow writing to object sysfs entry

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Date:	Tue, 20 Mar 2012 16:32:10 -0700
From:	Nick Dyer <nick.dyer@...ev.co.uk>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Daniel Kurtz <djkurtz@...omium.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Joonyoung Shim <jy0922.shim@...sung.com>,
	Iiro Valkonen <iiro.valkonen@...el.com>,
	Henrik Rydberg <rydberg@...omail.se>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Benson Leung <bleung@...omium.org>,
	Yufeng Shen <miletus@...omium.org>
Subject: Re: [PATCH 06/20] Input: atmel_mxt_ts - allow writing to object sysfs
 entry

Alan Cox wrote:
> On Tue, 13 Mar 2012 20:04:09 +0800
> Daniel Kurtz <djkurtz@...omium.org> wrote:
>> Userspace can write a 24-bit value (encoded as a 6 character hex string)
>> to the 'object' sysfs entry to modify a single byte of the object table.
>> The hex string encodes a 3 bytes, in the following format:
> 
> How is this locked against other users and updates of these table data ?
> 
> I don't see how all the objects and their pointers and size are
> guaranteedto always be valid in all the users you have ?

The particular layout of the "objects" on the maxtouch chip only changes if
there is a firmware upgrade (or between different models of chip). It's
just a convenience to allow bits of functionality to be extended or
replaced without getting in a mess of random register in different places,
but it's not dynamic.

In terms of arbitrating between different reads/writes - the I2C bus
locking will handle most of it. To be entirely correct, I think there
should be a mutex so that this interface couldn't be used whilst the chip
is resetting, backing up or upgrading firmware.

-- 
Nick Dyer
Software Engineer, ITDev Ltd
Hardware and Software Development Consultancy
Website: http://www.itdev.co.uk
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