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

From: Mark Brown <broonie@kernel.org>
To: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org, Hans de Goede <hdegoede@redhat.com>,
	linux-spi@vger.kernel.org, Martin Sperl <kernel@martin.sperl.org>,
	Michal Suchanek <hramrach@gmail.com>
Subject: Re: [PATCH] spi: Force the registration of the spidev devices
Date: Wed, 13 May 2015 12:26:04 +0100	[thread overview]
Message-ID: <20150513112604.GI3066@sirena.org.uk> (raw)
In-Reply-To: <1431462804-30467-1-git-send-email-maxime.ripard@free-electrons.com>

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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.

> Solve this by registering automatically spidev devices for all the unused chip
> selects when a master registers itself against the spi core.

So, aside from the concern about this being generic the other thing here
is that we often have devices offering more chip selects than can
physically be used in a system but not doing anything to ensure that the
invalid ones can't be used.  It's unclear to me if that's OK or not, it
might be since it's root only I think but I need to think it through a
bit.

> 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 here since it's a shared bus so you can't do
anything to devices not connected to the bus by mistake.

> +		/*
> +		 * This is far from perfect since an addition might be
> +		 * done between here and the call to spi_add_device,
> +		 * but we can't hold the lock and call spi_add_device
> +		 * either, as it would trigger a deadlock.
> +		 *
> +		 * If such a race occurs, spi_add_device will still
> +		 * catch it though, as it also checks for devices
> +		 * being registered several times on the same chip
> +		 * select.
> +		*/
> +		status = bus_for_each_dev(&spi_bus_type, NULL, spi,
> +					  spi_dev_check);
> +		if (status) {
> +			dev_dbg(&master->dev, "Chipselect already in use.. Skipping.");
> +			spi_dev_put(spi);
> +			continue;
> +		}

This still leaves us in the situation where if we do know the device
that is connected we have to explicitly bind it in spidev which is
apparently unreasonably difficult for people.  I'm also concerned about
the interactions with DT overlays here - what happens if a DT overlay
or other dynamic hardware instantiation comes along later and does bind
something to this chip select?  It seems like we should be able to
combine the two models, and the fact that we only create these devices
with a Kconfig option is a bit of an interesting thing here.

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