linux-kernel - Re: [linux-sunxi] [PATCH 2/3] spidev: Add DT binding example.

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Message-ID: <20150426125113.GF5627@lukather>
Date:	Sun, 26 Apr 2015 14:51:13 +0200
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Michal Suchanek <hramrach@...il.com>
Cc:	Martin Sperl <kernel@...tin.sperl.org>,
	Hans de Goede <hdegoede@...hat.com>,
	Mark Brown <broonie@...nel.org>,
	linux-sunxi <linux-sunxi@...glegroups.com>,
	Jonathan Corbet <corbet@....net>,
	linux-spi <linux-spi@...r.kernel.org>,
	linux-doc <linux-doc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [linux-sunxi] [PATCH 2/3] spidev: Add DT binding example.

On Sun, Apr 26, 2015 at 02:38:18PM +0200, Michal Suchanek wrote:
> On 26 April 2015 at 13:56, Martin Sperl <kernel@...tin.sperl.org> wrote:
> >
> >> On 26.04.2015, at 13:23, Hans de Goede <hdegoede@...hat.com> wrote:
> >> I think there is actual a use for just binding spidev as spidev,
> >> think e.g. the spi pins on the raspberry pi.
> >>
> >> How do you deal we suggest with such a situation ?
> >
> > I actually asked the same question a few days ago on the spi list
> > (in thread: "spi: spidev: Warn loudly if instantiated from DT as “spidev”)
> > and the summary was:
> >
> > You can still do as before, but you have to accept that long
> > irritating warning.
> >
> > Or you patch spidev.c to include your pattern of choice for compatiblity
> 
> So the suggestion is to add a compatible string like olimex,uext-slot
> to spidev and use that compatible in the DT?

No, you add a compatible for the device that is connected to the bus
through that slot.

> That can certainly be done but adding a new compatible for every board
> that has some random pins looks like a needless nuisance to me.
> Especially compared to i2c where you can just open the bus so long as
> ti is enabled.
> 
> >
> > Or you implement the following proposal (which needs a volunteer):
> >> On 23.04.2015, at 09:42, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> >>
> >> So what you need is a way to handover from generic spidev to a device-specific
> >> driver, cfr. what graphics drivers do when the device has been bound to by
> >> vesafb or simplefb.
> >>
> >> Could this be implemented in a generic way in the spi or DT code?
> >
> > ...
> >> On 23.04.2015, at 12:36, Mark Brown <broonie@...nel.org> wrote:
> >> On Thu, Apr 23, 2015 at 09:45:16AM +0200, Geert Uytterhoeven wrote:
> >>
> >>> I guess this has been suggested before: the spi core could provide spidev
> >>> access to all spi client devices which are not bound by a driver?
> >>
> >> I don't know if it's been suggested before, certainly nobody did the
> >> work to make it happen.  I don't think I have a massive objection in
> >> principal.

Actually, I did it a year ago, and it looked at the time that it
wasn't what should be done either.

https://lkml.org/lkml/2014/4/28/612

> But how do you know there is a device?
> 
> Devices on i2c can be probed. On spi you just transfer random data and
> hope it does something useful. Some devices have readable registers
> and can be probed in a device-specific way but others are write-only.

Well, what's the point of communicating with a non-existent device in
the first place?

> So binding spidev is in my view just saying that you are going to
> transfer random data from userspace on this bus.

Yes, to a device connected on that bus.

Maxime

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

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