[U-Boot] USB and unbinding

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 20-07-15 04:23, Simon Glass wrote:
> Hi Hans,
>
> I've been thinking about the USB unbinding code. I know that I agreed
> to go with it, but in retrospect I think that was a mistake.
>
> I believe we should separate out the unbinding and make it an option,
> so that it is not required in order to use USB. In effect this makes
> one of driver model's design goals (the option to drop unused code)
> useless since USB is a common interface.
>
> If I recall the only problem the lack of unbinding caused was that the
> keyboard driver broke. I suspect it broke in a way that can be fixed.
> In fact I recently converted usb_ether to driver model and I'm willing
> to do the keyboard side also.
>
> I'd like the USB code to function with or without the unbinding (i.e.
> it uses it if there). What do you think?

I strongly believe that unbinding is the proper thing todo for usb since
it is a hotplug bus.

IMHO the way the usb_find_emul_child() function was used before to re-use
udevice-s after e.g. a "usb reset" was an ugly hack which just happened to
work, but it in no way reflects reality.

More importantly we need unbind support to properly stop usb controllers when
booting the OS, so that they are not DMA-ing to/from their scratch-ram area
in DRAM when the main OS boots, so not having unbind support combined with
USB really is a no no.

This is why I suggested to simply select the unbind Kconfig when USB is
selected in Kconfig.

The actual unbind core code is not that big, so I believe that the best
solution is to always build the core if either DM_DEVICE_REMOVE *or*
DM_USB is selected, and non USB drivers can leave out their unbind
code if DM_DEVICE_REMOVE is not set, that should still give us most of
the size savings without needing to do ugly hacks for USB.

Regards,

Hans