On Sun, Apr 26, 2015 at 08:53:16PM +0200, Michal Suchanek wrote: > >> Also for driver prototyping you need a compatible which makes the > >> device accessible. > >> > >> If no spidev general compatible is available people will just use > >> compatible for some random device which happens to bind to spidev and > >> will send many letters of thanks to the DT maintainers when the device > >> used for this purpose suddenly grows a Linux driver. > > > > If people do dumb things, they should expect it to backfire. > > Yes, dumb things like not allowing people to say in the DT that the > board actually has pins on it connected to a SPI bus. Which is the > actual hardware which should be described in the DT. It's not connected to an SPI bus. It's connected to a device using an SPI bus. If you just had floating SPI lines, I'm pretty sure you wouldn't care about spidev at all. > Do you have to describe a modem or terminal emulator in DT to connect > it to your serial port? You just describe the port. So here you have a > SPI port and it should be described in the DT as faithfully as the > serial port. Except that in the serial port, you have a representation of a bus, while spidev represents a *device* connected on an SPI bus. So these are two different things, really. > >> >> > 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... > >> > >> I will if and when the device is usable. > > > > That's backward. The fact that your "driver" works really doesn't > > depend on what the device actually is. > > Indeed. > > However, for the device to have a compatible the compatible must be > specified in a driver and then I need a driver for the device to > record the compatible in. > > Or do you suggest that I patch the compatible into spidev, write a > driver for it, and then back out the compatible from spidev and check > in the compatible again with the driver? > > Now that is backwards. What Mark was suggesting was that you add a compatible to the spidev driver, and then you have access to spidev from userspace, period. If later on, you introduce a real driver for that, then yes, you would have to remove the compatible from spidev, and have that matching compatible in that new driver. Maxime -- Maxime Ripard, Free Electrons Embedded Linux, Kernel and Android engineering http://free-electrons.com
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