Re: [PATCH v4 1/2] char: xillybus: Move class-related functions to new xillybus_class.c

[Eli Billauer <eli.billauer@gmail.com>]

On 22/03/21 13:11, Greg KH wrote:
>
>> XILLYBUS and XILLYBUS_PCIE are currently enabled as M in several Linux
>> distributions. Making them depend on, rather than select XILLYBUS_CLASS is
>> likely to disable the driver in those distributions.
>>      
> That's not an issue here, depends-on will allow those distros to also
> enable this option.
>
> But wait, why is this a separate option at all?  Shouldn't the class
> code just be part of the "core" XILLYBUS code anyway?
>
>    
I'll try to explain the whole picture:

            XILLYBUS_CLASS
               /        \
              |          |
          XILLYBUS   XILLYUSB
       /           \
      |             |
XILLYBUS_PCIE  XILLYBUS_OF

XILLYBUS_CLASS is new and common to all drivers in this group.

XILLYBUS is for drivers based upon memory registers + DMA-based 
interfaces, and it's combined with XILLYBUS_PCIE and/or XILLYBUS_OF. 
XILLYUSB is for the USB variant only.

Or a more detailed, bottom-up outline:

* CONFIG_XILLYBUS_PCIE -> xillybus_pcie.c: Functions related to PCIe.
* CONFIG_XILLYBUS_OF -> xillybus_of.c: Functions related to Xillybus as 
a peripheral on an FPGA / Processor combo chip.
* CONFIG_XILLYBUS -> xillybus_core.c: Functions that are common to the 
two above, mainly access to the peripheral with memory-mapped registers 
and DMA.
* CONFIG_XILLYUSB -> xillyusb.c: The driver for the USB variant, 
accesses the peripheral through the USB framework.
* CONFIG_XILLYBUS_CLASS -> xillybus_class.c: The new module, which 
contains the class and API parts that would otherwise appear both in 
xillybus_core.c and xillyusb.c. Contains utility functions for the two 
latter.

Because XILLYBUS_CLASS is new and "N" by default, making a "depends on" 
relationship means that "make olddefconfig" silently turns off 
CONFIG_XILLYBUS and CONFIG_XILLYBUS_PCIE. That's a bug: Adding a new 
driver shouldn't change anything in the existing configuration.

That's why I had the "select XILLYBUS_CLASS" to begin with: It ensures a 
smooth transition on a kernel upgrade, by adding XILLYBUS_CLASS to 
kernels that had CONFIG_XILLYBUS enabled. Is there another way to 
prevent the said bug, without "select"?

Thanks and regards,
    Eli