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

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Message-ID: <CAKi4VAJACW5Wp4StHZQD6oPxs9KeqQt8fYy-AvtinfGFxErLzQ@mail.gmail.com>
Date:	Wed, 15 Jul 2015 03:27:33 -0300
From:	Lucas De Marchi <lucas.de.marchi@...il.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Mark Brown <broonie@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Hans de Goede <hdegoede@...hat.com>,
	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 7:33 PM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Wed, May 13, 2015 at 09:26:40PM +0200, Maxime Ripard wrote:
>> On Wed, May 13, 2015 at 11:17:36AM -0700, Greg Kroah-Hartman wrote:
>> > On Wed, May 13, 2015 at 07:50:34PM +0200, Maxime Ripard wrote:
>> > > Hi Greg,
>> > >
>> > > On Wed, May 13, 2015 at 08:37:40AM -0700, Greg Kroah-Hartman wrote:
>> > > > On Wed, May 13, 2015 at 12:26:04PM +0100, Mark Brown wrote:
>> > > > > On Tue, May 12, 2015 at 10:33:24PM +0200, Maxime Ripard wrote:
>> > > > >
>> > > > > > While this is nicer than the DT solution because of its accurate hardware
>> > > > > > representation, it's still not perfect because you might not have access to the
>> > > > > > DT, or you might be driving a completely generic device (such as a
>> > > > > > microcontroller) that might be used for something else in a different
>> > > > > > context/board.
>> > > > >
>> > > > > Greg, you're copied on this because this seems to be a generic problem
>> > > > > that should perhaps be solved at a driver model level - having a way to
>> > > > > bind userspace access to devices that we don't otherwise have a driver
>> > > > > for.  The subsystem could specify the UIO driver to use when no other
>> > > > > driver is available.
>> > > >
>> > > > That doesn't really work.  I've been talking to the ACPI people about
>> > > > this, and the problem is "don't otherwise have a driver for" is an
>> > > > impossible thing to prove, as you never know when a driver is going to
>> > > > be loaded from userspace.
>> > > >
>> > > > You can easily bind drivers to devices today from userspace, why not
>> > > > just use the built-in functionality you have today if you "know" that
>> > > > there is no driver for this hardware.
>> > >
>> > > What we're really after here is that we want to have an spidev
>> > > instance when we don't even have a device.
>> >
>> > That's crazy, just create a device, things do not work without one.
>>
>> Our use case is this one: we want to export spidev files so that "dev
>> boards" with a header that allows to plug virtually anything on it
>> (Raspberry Pi, Cubieboards, Xplained, and all the likes) without
>> having to change the kernel and / or device tree.
>
> You want to do that on a bus that is not self-describing or dynamic?
> I too want a pony.  Please go kick the hardware engineer who designed
> such a mess, we solved this problem 20+ years ago with "real" busses.

Mind you we are talking about buses created more than 20+ years ago.
(Unfortunately) They are still used today for all kind of sensors.
Boards like RPi, beaglebone, minnowboard expose the pins so we can
actually talk to those sensors, plugging in anyone we'd like to.  For
some of them for example there are IIO drivers that we could use the
driver model to allow binding them. But spidev/i2c-dev allow userspace
to talk directly to them. And you don't know what *others* will plug
into that bus... might even be their own microntrollers with no
identification at all.

Without something like the patch in the first message, people need to
create DT overlays for platforms that support that. ACPI doesn't
support overlays (yet) so we need to keep awful external platform
drivers[1] just to make spidev to work.

-- 
Lucas De Marchi

[1] https://github.com/MinnowBoard/minnow-max-extras/blob/master/modules/low-speed-spidev/low-speed-spidev.c
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