[PATCH] i2c: allow building emev2 without slave mode again

[Wolfram Sang]

On Thu, Dec 10, 2015 at 02:14:49PM +0100, Arnd Bergmann wrote:
> The emev2 driver stopped compiling in today's linux-next kernel:
> 
> drivers/i2c/busses/i2c-emev2.c: In function 'em_i2c_slave_irq':
> drivers/i2c/busses/i2c-emev2.c:233:23: error: storage size of 'event' isn't known
> drivers/i2c/busses/i2c-emev2.c:250:3: error: implicit declaration of function 'i2c_slave_event' [-Werror=implicit-function-declaration]
> drivers/i2c/busses/i2c-emev2.c:250:32: error: 'I2C_SLAVE_STOP' undeclared (first use in this function)
> 
> It works again if we enable CONFIG_I2C_SLAVE, but it seems wrong
> to add a dependency on that symbol:
> 
> * The symbol is user-selectable, but only one or two (including this
>   one) bus drivers actually implement it, and it makes no sense
>   if you don't have one of them.
> 
> * The other driver (R-Car) uses 'select I2C_SLAVE', which seems
>   reasonable in principle, but we should not do that on user
>   visible symbols.
> 
> * I2C slave mode could be implemented in a lot of other drivers
>   as an optional feature, but we shouldn't require enabling it
>   if we don't use it.
> 
> This changes the two drivers that provide I2C slave mode so they
> can again build if the slave mode being disabled. To do this, I
> move the definition of i2c_slave_event() and enum i2c_slave_event
> out of the #ifdef and instead make the assignment of the reg_slave
> and unreg_slave pointers optional in the bus drivers. The functions
> implementing the feature are unused in that case, so they get
> marked as __maybe_unused in order to still give compile-time
> coverage.

Thanks a lot! Making this clear and consistent was on my todo-list,
unfortunately below some other items.

Both drivers have quite orthogonal slave_irq routines. What do you think
about grouping this and the reg/unreg-calls together and compile them
conditionally on I2C_SLAVE? I think the code savings are worth it.

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