Linux Virtualization - Oct 2016 - [PATCH v9 17/19] drm/virtio: kconfig: Fix recursive dependency issue.

Hi Emil,

On Wed, 21 Sep 2016, Emil Velikov wrote:
> On 20 September 2016 at 09:32, Peter Griffin <peter.griffin at
linaro.org> wrote:
> > Hi Emil,
> >
> > On Tue, 20 Sep 2016, Emil Velikov wrote:
> >
> >> On 5 September 2016 at 14:16, Peter Griffin <peter.griffin at
linaro.org> wrote:
> >> > ST_SLIM_REMOTEPROC must select REMOTEPROC, which exposes the
following
> >> > recursive dependency.
> >
> >
> >> >
> >> From a humble skim through remoteproc, drm and a few other
subsystems
> >> I think the above is wrong. All the drivers (outside of
remoteproc),
> >> that I've seen, depend on the core component, they don't
select it.
> >
> > I will let Bjorn comment on the remoteproc subsystem Kconfig design,
and
> > why it is like it is.
> >
> > For this particular SLIM_RPROC I have added it to Kconfig in keeping
with all
> > the other drivers in the remoteproc subsystem which has exposed this
recursive
> > dependency issue.
> >
> > For this particular kconfig symbol a quick grep reveals more drivers
in
> > the kernel using 'select', than 'depend on'
> >
> > git grep "select VIRTIO" | wc -l
> > 14
> >
> > git grep "depends on VIRTIO" | wc -l
> > 10
> >
> Might be worth taking a closer look into these at some point.
VIRTIO has no dependencies, and is a non visible symbol. Its Kconfig help also
mentions that drivers should select it.
> 
> >
> >> Furthermore most places explicitly hide the drivers from the menu
if
> >> the core component isn't enabled.
> >
> > Remoteproc subsystem takes a different approach, the core code is only
enabled
> > if a driver which relies on it is enabled. This IMHO makes sense, as
> > remoteproc is not widely used (only a few particular ARM SoC's).
> >
> > It is true that for subsystems which rely on the core component being
> > explicitly enabled, they often tend to hide drivers which depend on it
> > from the menu unless it is. This also makes sense.
> >
> >>
> >> Is there something that requires such a different/unusual
behaviour in
> >> remoteproc ?
> >>
> >
> > I'm not sure it is that unusual...looking at config USB, it
selects USB_COMMON in
> > mfd subsystem, client drivers select MFD_CORE.
> >
> On the USB case I'm not sure what the reasoning behind the USB vs
> USB_COMMON split. In seems that one could just fold them, but that's
> another topic. On the MFD side... it follows the select {,mis,ab}use.
> With one (the only one?) MFD driver not using/selecting MFD_CORE doing
> it's own version of mfd_add_devices... which could be reworked,
> possibly.
Much like selecting VIRTIO in this patch, MFD_CORE is a non visible symbol
with no dependencies so it matches the documentation Jani referenced.

I personally think the approach taken makes sense, as why would you want to have
a CONFIG_MFD_CORE=y symbol being enabled unless you actually have a MFD device
which uses it also enabled in your kernel?

It seems to me a good solution to make the client drivers select the core
component so that it only gets enabled if at least one driver requires it.
This avoids having useless core code which will never be used compiled into the
kernel and in the end a smaller kernel size for a given kernel configuration
(better
cache use etc etc).
> > I've added Arnd to this thread, as I've seen lots of Kconfig
patches from him
> > recently, maybe he has some thoughts on whether this is the correct
fix or not?
> >
> Ack. Fwiw, I believe that the reasoning put by Jani is perfeclty
> reasonable, but it'll be great to hear others as well.
Yes me to. However I don't think anything in this patch is at odds with the
documentation Jani has referenced. 

regards,

Peter.

Hi Peter,

On 6 October 2016 at 11:48, Peter Griffin <peter.griffin at linaro.org>
wrote:
> Hi Emil,
>
> On Wed, 21 Sep 2016, Emil Velikov wrote:
>
>> On 20 September 2016 at 09:32, Peter Griffin <peter.griffin at
linaro.org> wrote:
>> > Hi Emil,
>> >
>> > On Tue, 20 Sep 2016, Emil Velikov wrote:
>> >
>> >> On 5 September 2016 at 14:16, Peter Griffin <peter.griffin
at linaro.org> wrote:
>> >> > ST_SLIM_REMOTEPROC must select REMOTEPROC, which exposes
the following
>> >> > recursive dependency.
>> >
>> >
>> >> >
>> >> From a humble skim through remoteproc, drm and a few other
subsystems
>> >> I think the above is wrong. All the drivers (outside of
remoteproc),
>> >> that I've seen, depend on the core component, they
don't select it.
>> >
>> > I will let Bjorn comment on the remoteproc subsystem Kconfig
design, and
>> > why it is like it is.
>> >
>> > For this particular SLIM_RPROC I have added it to Kconfig in
keeping with all
>> > the other drivers in the remoteproc subsystem which has exposed
this recursive
>> > dependency issue.
>> >
>> > For this particular kconfig symbol a quick grep reveals more
drivers in
>> > the kernel using 'select', than 'depend on'
>> >
>> > git grep "select VIRTIO" | wc -l
>> > 14
>> >
>> > git grep "depends on VIRTIO" | wc -l
>> > 10
>> >
>> Might be worth taking a closer look into these at some point.
>
> VIRTIO has no dependencies, and is a non visible symbol. Its Kconfig help
also
> mentions that drivers should select it.
>
This is a (un)fortunate detail cannot/should not overrule the other
arguments I've mentioned, should it ?
>>
>> >
>> >> Furthermore most places explicitly hide the drivers from the
menu if
>> >> the core component isn't enabled.
>> >
>> > Remoteproc subsystem takes a different approach, the core code is
only enabled
>> > if a driver which relies on it is enabled. This IMHO makes sense,
as
>> > remoteproc is not widely used (only a few particular ARM
SoC's).
>> >
>> > It is true that for subsystems which rely on the core component
being
>> > explicitly enabled, they often tend to hide drivers which depend
on it
>> > from the menu unless it is. This also makes sense.
>> >
>> >>
>> >> Is there something that requires such a different/unusual
behaviour in
>> >> remoteproc ?
>> >>
>> >
>> > I'm not sure it is that unusual...looking at config USB, it
selects USB_COMMON in
>> > mfd subsystem, client drivers select MFD_CORE.
>> >
>> On the USB case I'm not sure what the reasoning behind the USB vs
>> USB_COMMON split. In seems that one could just fold them, but
that's
>> another topic. On the MFD side... it follows the select {,mis,ab}use.
>> With one (the only one?) MFD driver not using/selecting MFD_CORE doing
>> it's own version of mfd_add_devices... which could be reworked,
>> possibly.
>
> Much like selecting VIRTIO in this patch, MFD_CORE is a non visible symbol
> with no dependencies so it matches the documentation Jani referenced.
>
> I personally think the approach taken makes sense, as why would you want to
have
> a CONFIG_MFD_CORE=y symbol being enabled unless you actually have a MFD
device
> which uses it also enabled in your kernel?
>
> It seems to me a good solution to make the client drivers select the core
> component so that it only gets enabled if at least one driver requires it.
> This avoids having useless core code which will never be used compiled into
the
> kernel and in the end a smaller kernel size for a given kernel
configuration (better
> cache use etc etc).
>
>> > I've added Arnd to this thread, as I've seen lots of
Kconfig patches from him
>> > recently, maybe he has some thoughts on whether this is the
correct fix or not?
>> >
>> Ack. Fwiw, I believe that the reasoning put by Jani is perfeclty
>> reasonable, but it'll be great to hear others as well.
>
> Yes me to. However I don't think anything in this patch is at odds with
the
> documentation Jani has referenced.
>
It case it's not clear, let me elaborate:

Yes, using depend might not be the most user-friendly thing to do and
in the long term we might want to move to select.
Yes, I agree with the argument about symbol visibility but that is not
the only contributing factor.

If one wants to pick on specific users which opt for $driver select
$core they must do the same for $driver depends on $core. Check the
number 'in favour" of each case and draw their conclusions ;-)

In particular: both MFD_CORE and USB_COMMON, are _optional_ as only
some drivers depends on them. Thus giving them as an example is
wrong/irrelevant, I'm afraid.

Thanks
Emil