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

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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
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