Date:	Thu, 5 Jun 2014 15:59:59 +0200
From:	Tom Gundersen <teg@...m.no>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Takashi Iwai <tiwai@...e.de>, Stefan Roese <sr@...x.de>,
	Arnd Bergmann <arnd@...db.de>,
	Abhay Salunke <Abhay_Salunke@...l.com>,
	Kay Sievers <kay@...y.org>
Subject: Re: [PATCH v2] firmware loader: allow disabling of udev as firmware loader

On Thu, Jun 5, 2014 at 3:31 PM, Ming Lei <ming.lei@...onical.com> wrote:
> On Thu, Jun 5, 2014 at 8:25 PM, Tom Gundersen <teg@...m.no> wrote:
>>
>> On 5 Jun 2014 14:18, "Ming Lei" <ming.lei@...onical.com> wrote:
>>>
>>> On Wed, Jun 4, 2014 at 11:48 PM, Takashi Iwai <tiwai@...e.de> wrote:
>>> > [The patch was originally proposed by Tom Gundersen, and rewritten
>>> >  afterwards by me; most of changelogs below borrowed from Tom's
>>> >  original patch -- tiwai]
>>> >
>>> > Currently (at least) the dell-rbu driver selects FW_LOADER_USER_HELPER,
>>> > which means that distros can't really stop loading firmware through
>>> > udev without breaking other users (though some have).
>>> >
>>> > Ideally we would remove/disable the udev firmware helper in both the
>>> > kernel and in udev, but if we were to disable it in udev and not the
>>> > kernel, the result would be (seemingly) hung kernels as no one would
>>> > be around to cancel firmware requests.
>>> >
>>> > This patch allows udev firmware loading to be disabled while still
>>> > allowing non-udev firmware loading, as done by the dell-rbu driver, to
>>> > continue working. This is achieved by only using the fallback
>>> > mechanism when the uevent is suppressed.
>>> >
>>> > The patch renames the user-selectable Kconfig from FW_LOADER_USER_HELPER
>>> > to FW_LOADER_USER_HELPER_FALLBACK, and the former is reverse-selected
>>> > by the latter or the drivers that need userhelper like dell-rbu.
>>> >
>>> > Also, the "default y" is removed together with this change, since it's
>>> > been deprecated in udev upstream, thus rather better to disable it
>>> > nowadays.
>>> >
>>> > Tested with
>>> >     FW_LOADER_USER_HELPER=n
>>> >     LATTICE_ECP3_CONFIG=y
>>> >     DELL_RBU=y
>>> > and udev without the firmware loading support, but I don't have the
>>> > hardware to test the lattice/dell drivers, so additional testing would
>>> > be appreciated.
>>> >
>>> > Reviewed-by: Tom Gundersen <teg@...m.no>
>>> > Cc: Ming Lei <ming.lei@...onical.com>
>>> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>> > Cc: Abhay Salunke <Abhay_Salunke@...l.com>
>>> > Cc: Stefan Roese <sr@...x.de>
>>> > Cc: Arnd Bergmann <arnd@...db.de>
>>> > Cc: Kay Sievers <kay@...y.org>
>>> > Signed-off-by: Takashi Iwai <tiwai@...e.de>
>>> > ---
>>> >  drivers/base/Kconfig          | 10 ++++++++--
>>> >  drivers/base/firmware_class.c | 15 ++++++++++-----
>>> >  include/linux/firmware.h      |  2 +-
>>> >  3 files changed, 19 insertions(+), 8 deletions(-)
>>> >
>>> > diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
>>> > index 8fa8deab6449..d0bb32e4c416 100644
>>> > --- a/drivers/base/Kconfig
>>> > +++ b/drivers/base/Kconfig
>>> > @@ -144,15 +144,21 @@ config EXTRA_FIRMWARE_DIR
>>> >           some other directory containing the firmware files.
>>> >
>>> >  config FW_LOADER_USER_HELPER
>>> > +       bool
>>> > +
>>> > +config FW_LOADER_USER_HELPER_FALLBACK
>>> >         bool "Fallback user-helper invocation for firmware loading"
>>> >         depends on FW_LOADER
>>> > -       default y
>>> > +       select FW_LOADER_USER_HELPER
>>> >         help
>>> >           This option enables / disables the invocation of user-helper
>>> >           (e.g. udev) for loading firmware files as a fallback after the
>>> >           direct file loading in kernel fails.  The user-mode helper is
>>> >           no longer required unless you have a special firmware file
>>> > that
>>> > -         resides in a non-standard path.
>>> > +         resides in a non-standard path. Moreover, the udev support has
>>> > +         been deprecated upstream.
>>> > +
>>> > +         If you are unsure about this, say N here.
>>>
>>> It may be safer to say Y here for fallback if not sure.
>>
>> Saying Y here will be actively harmful if the firmware loader is disabled in
>> udev (which I guess most distros will do as soon as they can), so I think we
>
> If fallback is triggered, it means that the firmware can't be found
> in default direct rootfs path, so it is better to continue to look for it
> from userspace.

The kernel looks in the same paths as udev does, so unless you are
doing something very special (which should mean you know what you are
doing), saying N will not hurt.

> Also it won't a big problem for hanging the request user context
> for some while(60sec at default) if udev is disabled, will it?

Things will still work, but I'm sure we would get a lot of
confused/angry users. After all, the 60 second hangs were precisely
the reason the firmware loader was moved to the kernel in the first
place.

> BTW, are you sure most distros will do that in the near future?

There is no reason not to, and upstream udev will certainly remove the
feature very soon (so anyone who wants it would have to patch it back
in).

>> should advice people to say N unless they really know what they are doing
>> and that their userspace can cope with it.
>
> That is why I suggest to say Y if someone isn't sure.

I don't follow. Saying N will always be safe unless you have a very
custom setup, saying Y will most likely cause lots of confusing hangs
unless you really have made sure that your userland is set up
accordingly...

Cheers,

Tom
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