linux-kernel - Re: [PATCH] spi: Force the registration of the spidev devices

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Message-ID: <20150513194106.GG4004@lukather>
Date:	Wed, 13 May 2015 21:41:06 +0200
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Mark Brown <broonie@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Hans de Goede <hdegoede@...hat.com>,
	linux-spi <linux-spi@...r.kernel.org>,
	Martin Sperl <kernel@...tin.sperl.org>,
	Michal Suchanek <hramrach@...il.com>
Subject: Re: [PATCH] spi: Force the registration of the spidev devices

On Wed, May 13, 2015 at 09:10:48PM +0200, Geert Uytterhoeven wrote:
> On Wed, May 13, 2015 at 2:51 PM, Maxime Ripard
> <maxime.ripard@...e-electrons.com> wrote:
> >> > This also adds an i2cdev-like feeling, where you get all the
> >> > spidev devices all the time, without any modification.
> >>
> >> I2C is a bit safer here since it's a shared bus so you can't do
> >> anything to devices not connected to the bus by mistake.
> >
> > I'm not sure to understand what you mean here. How is SPI different
> > from that aspect?
> 
> If you talk to a nonexistent i2c device, nothing happens, as it just sends
> a message with a nonexistent address on the shared bus.
> 
> If you talk to a nonexistent spi device, hell may break loose if e.g. some
> "smart" hardware engineer used the "unused" CS as a pull-up for the
> _RESET line on an external device... It's a bit like banging random
> "unused" GPIOs.

Ah, right. I'm always surprised by how creative the hardware engineers
actually are :)

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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