Re: [PATCH 2/3] spidev: Add DT binding example.

[Maxime Ripard <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>]

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On Sun, Apr 26, 2015 at 04:14:33PM +0200, Michal Suchanek wrote:
> On 26 April 2015 at 14:51, Maxime Ripard
> <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org> wrote:
> > On Sun, Apr 26, 2015 at 02:38:18PM +0200, Michal Suchanek wrote:
> >> On 26 April 2015 at 13:56, Martin Sperl <kernel-TqfNSX0MhmxHKSADF0wUEw@public.gmane.org> wrote:
> >> >
> >> >> On 26.04.2015, at 13:23, Hans de Goede <hdegoede-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 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.
> 
> There is no device connected in the slot by design. The slot is there
> for connecting random stuff you find in your mailbox or other drawers
> and boxes.

I know. Our point is add a compatible for that random device you find
in your mailbox.

> >> 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-Td1EMuHUCqxL1ZNQvxDV9g@public.gmane.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-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.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.
> 
> There is nothing like unclaimed device. Either there is a device and
> driver for it may in principle be loaded later as a module or the chip
> select is reserved for use from userspace.

I never said it was perfect.

> Userspace driver is valid option and should have the ability to have
> the chip select reserved.

Whether an userspace driver is a valid option can spawn a whole debate
by its own, but it's true that we should be able to have it exported
to the userspace.

However, having a spidev compatible is not the solution to that
problem.

> > 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?
> 
> I have multitude of SPI devices which are not part of the board and
> hence its DT and can be connected to the board with jumper wires.
> 
> Most of them don't have a linux driver or compatible to bind with.

Then create such a compatible...

> >> 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.
> 
> Yes, so I have this red rectangular PCB, blue PCB, and red square PCB
> and blue very thin rectangular PCB.
> 
> Please enlighten me how to add DT bindings for these and the PCB which
> is in the mail and I did not pick up at the post office yet.
> 
> Of course, I have some hope that the chips on the PCBs are at least
> somewhat compatible with what I ordered but I cannot be sure until I
> test that.

Come on, this is pure bad faith. If you don't even know what that
device is, how do you even know how to interact with it?

Maxime

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

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