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 07:51:44 -0700
From:	Nick Dyer <nick.dyer@...ev.co.uk>
To:	Daniel Kurtz <djkurtz@...omium.org>
CC:	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>,
	"Bowens, Alan" <Alan.Bowens@...el.com>,
	"Tiwari, Atul" <Atul.Tiwari@...el.com>
Subject: Re: [PATCH 06/20] Input: atmel_mxt_ts - allow writing to object sysfs
 entry

Daniel Kurtz 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:
>  TTFFVV
> 
> Where:
>  TT = object type
>  FF = object offset
>  VV = byte value
> 
> The object table is modified in device ram, which means the change is
> volatile, and will be overwritten on the next device reset.  To make
> changes permanent, the new settings should be persisted in the device's
> Non-Voltatile Memory using the updatenv sysfs entry.
> 
> Also, since the device driver initializes itself by reading some values
> from the object table, the entire driver may need to be unloaded and
> reloaded after writing the values for the driver to stay in sync.  Whether
> this is required depends on exactly which values were updated.
> 
> Signed-off-by: Daniel Kurtz <djkurtz@...omium.org>

NAK. I have several concerns about this:

1) The object sysfs entry doesn't follow the sysfs guidelines, this is
abusing it further.
2) Object type can be larger than 1 byte in the future.
3) Your interface restricts you to doing 1 byte writes. What about larger
block sizes?

In short, I think exposing the entire register space as a binary attribute
(the same way that regmap does) is a preferable solution to this
requirement, and we already have user space tools which use it.

cheers

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