Hi Paul, On Mon, Dec 20, 2021 at 1:07 PM Paul Menzel <pmenzel@xxxxxxxxxxxxx> wrote: > > From: Furquan Shaikh <furquan@xxxxxxxxxx> > > Dear Linux folks, > > > Google Chromebooks are often built with devices sourced from different > vendors. These need to be probed. To deal with this, the firmware – in > this case coreboot – tags such optional devices accordingly – I think > this is commit fbf2c79b (drivers/i2c/generic: Add config for marking > device as probed) – and Chromium OS’ Linux kernel has the patch at hand > applied to act accordingly. Right after the merge, Dmitry created a > revert, which was actively discussed for two days but wasn’t applied. > That means, millions of devices shipped with such a firmware and Linux > kernel. To support these devices with upstream Linux kernel, is there an > alternative to applying the patch to the Linux kernel, and to support > the shipped devices? *sigh* I should have pushed harder, but I see it managed to proliferate even into our newer kernels. Not having this patch should not cause any problems, it can only hurt, because the i2c core has no idea how to power up and reset the device properly. The only downside of not having this patch is that we may have devices in sysfs that are not connected to actual hardware. They do now cause any problems and is how we have been shipping ARM-based devices where we also dual- and triple-source components. However if we were to have a device that switches between several addresses (let's say device in bootloader mode uses 0x10 address and in normal mode 0x20) this "probing" may result in device not being detected at all. If we wanted to do this correctly, coreboot would have to implement full power and reset control and also add drivers for I2C controllers to be able to communicate with peripherals, and then adjust _STA methods to report "not present" when the device is indeed absent. And note that even in this case we would have issues with "morphing devices", so coreboot would also need to know how to reset device out of bootloader mode, and maybe flash firmware so device can work in normal mode. However coreboot does (or did?) not want to add code to handle i2c controllers, and would like to push this knowledge to the kernel. And the kernel does know how to handle peripherals properly, but that knowledge lies in individual drivers, not i2c core. We should remove "linux,probed" from coreboot and not propagate to newer Chrome OS kernels, and keep it away from upstream. Thanks, Dmitry
