Re: [PATCH] binder: let ANDROID_BINDER_IPC_32BIT be selectable on 32bit ARM

[John Stultz]

On Tue, Aug 22, 2017 at 8:01 PM, Jisheng Zhang <jszhang@xxxxxxxxxxx> wrote:
> On Tue, 22 Aug 2017 19:57:04 -0700 John Stultz wrote:
>
>> On Tue, Aug 22, 2017 at 7:56 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
>> > On Tue, Aug 22, 2017 at 7:34 PM, Jisheng Zhang <jszhang@xxxxxxxxxxx> wrote:
>> >> On Tue, 22 Aug 2017 18:51:08 -0700 Greg KH wrote:
>> >>
>> >>> On Tue, Aug 08, 2017 at 07:03:05PM +0800, Jisheng Zhang wrote:
>> >>> > As noted in commit d0bdff0db809 ("staging: Fix build issues with new
>> >>> > binder API"), we can add back the choice for 32bit ARM "once a 64bit
>> >>> > __get_user_asm_* implementation is merged." Commit e38361d032f1 ("ARM:
>> >>> > 8091/2: add get_user() support for 8 byte types") has added the
>> >>> > support, so it's time to let ANDROID_BINDER_IPC_32BIT be selectable on
>> >>> > 32bit ARM
>> >>>
>> >>> Ok, but:
>> >>>
>> >>> >
>> >>> > Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxxx>
>> >>> > ---
>> >>> >  drivers/android/Kconfig | 2 +-
>> >>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >>> >
>> >>> > diff --git a/drivers/android/Kconfig b/drivers/android/Kconfig
>> >>> > index 832e885349b1..aca5dc30b97b 100644
>> >>> > --- a/drivers/android/Kconfig
>> >>> > +++ b/drivers/android/Kconfig
>> >>> > @@ -32,7 +32,7 @@ config ANDROID_BINDER_DEVICES
>> >>> >       therefore logically separated from the other devices.
>> >>> >
>> >>> >  config ANDROID_BINDER_IPC_32BIT
>> >>> > -   bool
>> >>> > +   bool "Use old (Android 4.4 and earlier) 32-bit binder API"
>> >>> >     depends on !64BIT && ANDROID_BINDER_IPC
>> >>>
>> >>> You don't actually change the depends line :(
>> >>>
>> >>> Please fix up, and test it, and then resend.
>> >>
>> >> IHOM, the dependency is correct: 64bit platforms don't support
>> >> ANDROID_BINDER_IPC_32BIT. What do you think?
>> >
>> > I think this indicates the commit message is unclear.
>> >
>> > Part of it is that the config is inverted from the description. The
>> > patch doesn't enable the 32bit legacy binder ABI on 32bit systems, it
>> > just allows the option to be unselected, so that the 64bit ABI will be
>> > used on 32bit systems.
>> >
>> > Conceptually I don't have an objection to the change (though maybe try
>> > to rework the commit message), but I don't have anything to actually
>> > test it on right now, so I'm hesitant to ack it.
>>
>> It might also be good to add some detail as to the motivation for this
>> change? What benefit does it bring to 32bit platforms to use the newer
>> 64bit ABI?
>>
>
> To be honest, the motivation is just to add one more choice for 32bit
> platform and let the code be tested under 32bit platform. Maybe we
> could then remove ANDROID_BINDER_IPC_32BIT and the related code after
> some time?

I'm mixed. It would be nice to deprecate the old 32bit ABI, but binder
is a real Linux kernel interface now, so we don't break compatibility
(at least if it affects anyone - which may be questionable here - not
sure there's many upstream 32bit platforms concerned with running
legacy Android builds).  But just adding the extra option just means
there's yet another configuration to test and to keep working. So you
may want to articulate the benefits better to make this worth the
effort of doing a full transition.

thanks
-john