[U-Boot] USB and unbinding

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

Hi,

On 07/20/2015 05:49 PM, Simon Glass wrote:
> Hi Hans,
>
> On 20 July 2015 at 09:31, Hans de Goede <hdegoede@redhat.com> wrote:
>> 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.
>
> I think you are referring to remove(), not unbind().

Right I mean that the remove callback *must* be called on usb_stop to avoid
the usb controller dma-ing over random DRAM when the OS starts.

> Although we might
> consider spiting them so we have a DM_DEVICE_REMOVE and a separate
> DM_DEVICE_UNBIND.
>
>>
>> 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.
>
> My main objection is that we tie USB such that it *will not work*
> unless we support unbinding. I'm fine with it being recommended, but
> core driver model features should be independent of subsystems. This
> also seems quite unnecessary. Re your common about the 'ugly hack that
> just happened to work', in principle we can just keep on creating new
> devices and ignore the old ones.

That will still cause problems with code addressing the usb-devices
by index, as the old devices will still be there.

> That's the idea behind not supporting
> unbinding. There should be no problem with this approach.

This approach will only work if find_child_devnum is fixed to search
backwards through the childs list, so that it will check the newly
added nodes first.

> So I'd like to adjust the USB code so that it still works without
> unbinding, even if it is not optimal. I think that is the right thing
> to do in this case.

As said, the remove callback of usb-host drivers *must* always be called,
other then that if you can make things work without unbind that is
fine with me.

Regards,

Hans