linux-kernel - Re: [PATCH RFC] spidev.c: add sysfs attributes for SPI configuration

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Message-Id: <20121222094725.797F03E03CE@localhost>
Date:	Sat, 22 Dec 2012 09:47:25 +0000
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Federico Vaga <federico.vaga@...il.com>
Cc:	spi-devel-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] spidev.c: add sysfs attributes for SPI configuration

On Thu, 20 Dec 2012 16:30:36 +0100, Federico Vaga <federico.vaga@...il.com> wrote:
> On Wednesday 19 December 2012 15:09:25 Grant Likely wrote:
> > Not a good idea. sysfs is not a good place for operational
> > interfaces. Please use the spi character devices for direct
> > manipulation of the SPI configuration.
> 
> Hello,
> 
> Can you explain why it is not a good idea? I do not understand; what 
> is the advantage of ioctl through char device? Or what it the issue 
> with sysfs?
> 
> Thank you very much

I'm cautious about adding operational interfaces to sysfs because it can
be quite difficult to get the locking right. To begin with it splits up
a single interface into multiple files, any of which can be held open by
a process. Then there is the question of ordering of operations when
there are multiple users. For instance, if there were two users, each of
which using different transfer parameters, a sysfs interface doesn't
provide any mechanism to group setting up the device with the transfer.

These are lessons learned the hard way with the gpio sysfs abi. I don't
want to get caught in the same trap for spi.

g.
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