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

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Date:	Mon, 5 May 2014 09:10:43 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Mark Brown <broonie@...nel.org>,
	linux-spi <linux-spi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
Subject: Re: [PATCH] spi: Force the registration of the spidev devices

Hi Maxime,

On Mon, May 5, 2014 at 6:17 AM, Maxime Ripard
<maxime.ripard@...e-electrons.com> wrote:
> On Fri, May 02, 2014 at 01:28:26AM +0200, Geert Uytterhoeven wrote:
>> On Fri, May 2, 2014 at 12:36 AM, Maxime Ripard
>> <maxime.ripard@...e-electrons.com> wrote:
>> > But it actually doesn't work in a case where you can't really predict
>> > what is on the other side of the bus. Either because, on the board
>> > you're using the pins are exposed and it's pretty much up to the user
>> > to know what to put on it. That could be handled by DT overlays
>> > though.
>> >
>> > What never works is where the device on the other side is so generic
>> > that you really can't tell what it does. Think of a microcontroller
>> > that would behave as a SPI slave. It's behaviour and what it does is
>> > pretty much dependant of what we flashed on it, and suddenly the
>> > compatible string is not the proper reprensentation anymore.
>>
>> So you will (hopefully soon) use overlay DT to change the DTS to match what's
>> connected?
>
> Not really, because you can't declare a spidev device in the DT.

Yes you can. I've done it before.

See also "git grep -w spidev -- arch/arm/*/dts/".

>> And then you want spidev to bind to it. Would it help if DT offered a feature
>> to add a compatible entry to a driver at runtime, cfr.
>> /sys/bus/pci/drivers/.../new_id on PCI?
>
> The thing is, in the core SPI bus, devices are only instiated (in the
> DT case), by parsing the DT. So, to bind a device to a new driver, it
> has to exist in the first place, and it won't exist, because there
> won't be any node in the DT for this device. But since DT is
> unsuitable for such a device, you go back to where we were initially.

So you add a node in the overlay DT? Or what am I missing?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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