Re: [RESEND PATCH -net] cpsw/netcp: cpts depends on posix_timers

On Mon, 13 Mar 2017, Arnd Bergmann wrote:

On Mon, Mar 13, 2017 at 9:09 PM, Nicolas Pitre [off-list ref] wrote:

quoted

On Mon, 13 Mar 2017, Arnd Bergmann wrote:

quoted

With posix timers having become optional, we get a build error with
the cpts time sync option of the CPSW driver:

drivers/net/ethernet/ti/cpts.c: In function 'cpts_find_ts':
drivers/net/ethernet/ti/cpts.c:291:23: error: implicit declaration of function 'ptp_classify_raw';did you mean 'ptp_classifier_init'? [-Werror=implicit-function-declaration]

It really makes no sense to build this driver if we can't use PTP,
so it's better to go back to 'select PTP_1588_CLOCK' but instead
add a dependency on POSIX_TIMERS. Adding 'depends on PTP_1588_CLOCK'
might also work, but has the risk of circular dependencies when
mixed with other drivers using 'imply'.

Could you elaborate on that risk please?

I have seen many circular dependencies in the past that tend to be of type

config FOO
     depends on A
     select B

config BAR
     select A
     depends on B

This is really a problem?  I mean in this example there is nothing that 
prevents A or B to be enabled independently.  Of course if you had:

config A
	depends on B

config B
	depends on A

then the circular dependency is obvious.

The best way to avoid this problem is to only ever use either 'select' or
'depends on' for any given dependency, but not both. In this case, almost
all references to PTP_1588_CLOCK use 'select' or 'implies', so I don't
want to introduce any more 'depends on'.

I can't find any "select PTP_1588_CLOCK" in the tree.

The "imply" keyword in itself doesn't create nor inforce any 
dependencies -- that's why it was created in the first place.

So unless I'm mistaken I don't see any problem using "depends on 
PTP_1588_CLOCK" here.


Nicolas