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

From: Maxime Ripard
Date: Wed May 13 2015 - 15:10:14 EST

On Wed, May 13, 2015 at 03:36:10PM +0100, Mark Brown wrote:
> On Wed, May 13, 2015 at 02:51:02PM +0200, Maxime Ripard wrote:
> > I'd say we're also ok because if we delegate the device driving logic
> > to userspace, we should expect it to know what it does to first drive
> > the device properly, but also to open the right device for this.
> > What's the worst that could happen in such a case? The data are output
> > without any chipselect line being driven by the controller? Isn't that
> > supposed to be ignored by the devices?
> I'm more worried about the chip select line being connected to the
> "make the board catch fire" signal or whatever (more realistically
> causing us to drive against some other external component) if the extra
> chip selects weren't pinmuxed away.

It seems we've had this discussion at lot lately ;)

That indeed might be problematic....

> > > > 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?
> Chip select signals.

Well, if it's not connected to the bus, it probably won't be connected
to the chip select either, will it?

> > > 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.
> > You can still do that, but the point is that you don't have to.
> Right, but that's not what I'd expect to happen (and seems to make it
> easier for people to not list things in the DT at all which doesn't seem
> great). If we're going to make it available by default I'd expect to be
> able to use a userspace driver with anything that doesn't have a driver
> bound rather than with devices that explicitly don't have any
> identification.

The point is that if we don't have anything declared in the DT, we
won't even have a device. So we can't really expect that the device
will not be bound to a driver, because it won't even be there in the
first place.

> > > 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.
> > I think the safe approach would be, just like I told in this thread,
> > to just check whether the modalias is spidev. If it is, destroy the
> > previous (spidev) device, create a new device as specified by the DT,
> > you're done.
> Sure, but I don't see code for that here.

No, of course. Remember that this code was written before the overlays
were posted.


Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering

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