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Subjectfpga: fpga_mgr_free usage
Date
Hi Alan,

I have another point that I would like to discuss. It is about the
usage of 'fpga_mgr_free()' which does not look like consistent.

This function, according to the current implementation, can be used by
an FPGA manager user and it is used by the FPGA manager itself on
device release. This means that the user can only use this function if
fpga_mgr_register() fails (to clean up), otherwise the user must NOT
use this function, otherwise we most likely get an oops (NULL or
invalid pointer). The example here is correct, this is what we should
do:

https://www.kernel.org/doc/html/latest/driver-api/fpga/fpga-mgr.html

But I suggest to document it better or prevent this type of mistakes
from happening. Following a couple of proposals

------
1.
Document it better. This is easy, in the fpga_mgr_free() kernel-doc
comment we explain that the use of this function must be limited to
clean up the memory on a registration failure. If an FPGA manager has
been successfully registered then it will be freed by the framework
itself.

But still, this does not prevent an oops from happening.
------
2.
Remove the fpga_mgr_free() from fpga_mgr_dev_release() and ask the
user to free the manager when necessary.

This makes the usage consistent: the user creates and destroy its own
objects. This is also consistent with our other discussion where we
said, among the other things, that the module that uses the FPGA
manager can the owner of the fpga_manager data structure.
------
3.
Not sure how, but perhaps we can be able to understand if we can
safely continue to run fpga_mgr_free() or we should stop.
------


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