[PATCH] imx_v6_v7_defconfig: enable SPIDEV module

[Gary Bisson]

Hi Javier,

Thanks for the quick feedback on the matter.

On Fri, Dec 02, 2016 at 12:44:42PM -0300, Javier Martinez Canillas wrote:
> Hello Gary,
> 
> On Fri, Dec 2, 2016 at 12:27 PM, Gary Bisson
> <gary.bisson at boundarydevices.com> wrote:
> > Hi Fabio,
> >
> > On Fri, Dec 02, 2016 at 01:18:42PM -0200, Fabio Estevam wrote:
> >> Hi Gary,
> >>
> >> On Fri, Dec 2, 2016 at 12:56 PM, Gary Bisson
> >> <gary.bisson at boundarydevices.com> wrote:
> >> > Signed-off-by: Gary Bisson <gary.bisson at boundarydevices.com>
> >> > ---
> >> > Hi,
> >> >
> >> > Seems that this configuration is missing, especially since many
> >> > device trees are already using "spidev" nodes.
> >>
> 
> This seems to have been added just because people weren't looking that
> closer to DT patches at the beginning, but now is forbidden. That's
> why the kernel now warns about it.
> 
> >> Then they will trigger the following warning below (from the spidev
> >> probe function):
> >>
> >> /*
> >> * spidev should never be referenced in DT without a specific
> >> * compatible string, it is a Linux implementation thing
> >> * rather than a description of the hardware.
> >> */
> >> if (spi->dev.of_node && !of_match_device(spidev_dt_ids, &spi->dev)) {
> >>     dev_err(&spi->dev, "buggy DT: spidev listed directly in DT\n");
> >>     WARN_ON(spi->dev.of_node &&
> >>                        !of_match_device(spidev_dt_ids, &spi->dev));
> >> }
> >
> > Yes I've seen this WARN_ON when doing the dt-overlay testing. But is
> > disabling the SPIDEV configuration a solution?
> >
> > To be honest I disagree with that WARN_ON. Ok it means that the hardware
> > isn't fully described in the device tree but for development platforms
> > (such as ours or any rpi-like boards) the user can design his own
> > daugher board with whatever SPI device on it. Then using the spidev
> > interface is very convenient, so I don't understand what we are trying
> > to forbid here.
> >
> 
> AFAICT, what we are trying to forbid is to have a Linux implementation
> detail to creep into a Device Tree.
> 
> It's OK to use the spidev interface but that's orthogonal on how the
> device is instantiated from the DT. If you want to do that, the
> correct approach is AFAIU to add a OF device ID entry in
> drivers/spi/spidev.c and use that compatible string in your DT.

I understand that the device tree isn't supposed to describe such
generic "spidev" concept.

And that argument to me is ok for final products where the hardware is
fully defined. However I still believe that for development platforms
this cannot be applied. My customers want to use the SPI bus to connect
any device they want, and most of those customers don't want to mess
with the kernel. For them, having a spidev node, just like it exists for
i2cdev nodes, is ideal.

> That way, the DT will describe the hardware instead of just a "spidev"
> but you could use the spidev interface to access your SPI device.

But then aren't you afraid that the person will use the first compatible
that shows up (let's say "rohm,dh2228fv") and use it for all his spidev?
In that case the driver is happy although the fact remains that any hw
will be plugged behind. Or at the opposite, if everybody that uses
spidev adds its own compatible I'm not sure it will benefit the drive
code.

Anyway, I appreciate your feedback.

Regards,
Gary