[elrepo] Downgrading between kernel versions that use kmod-megaraid_sas

[Roger Sewell]

Having successfully got a test kernel to run using kmod-megaraid_sas,
I'm now having difficulty working out how to downgrade to my previous
kernel needing the same driver.

(I do have a backup disc image, so I'm not up a creek without a paddle,
but I want to understand how to do it properly and whether or not I'm
experiencing a bug.)

I originally had running 

kernel-4.18.0-193.6.3.el8_2.x86_64 and
kmod-megaraid_sas-07.710.50.00-1.el8_2.elrepo.x86_64 . 

I was sent

kernel-4.18.0-287.el8.test.x86_64

and its associated devel etc packages. I compiled kmod-megaraid_sas from
its 8.3 src package for this new kernel giving

kmod-megaraid_sas-07.714.04.00-1r.el8_3.test.x86_64.rpm 

which installed successfully. Apart from unrelated issues this new
kernel ran OK. However, with this setup, the original kernel wouldn't
run any more, instead it hung after the first occurrence of "Reached
target Basic System" in boot.log .

Now I want to downgrade to the original kernel and kmod-megaraid_sas
package. My first instinct was to simply do

yum --cacheonly downgrade kmod-megaraid_sas-07.710.50.00-1.el8_2.elrepo.x86_64.rpm

(providing that package in the current directory) thinking that this
would result in the 8_2 package in place (it did) and the 8_2 kernel now
working (with the test kernel now not working).

However, instead, both kernels hang at the first occurrence of "Reached
target Basic System", before finding the root device.

Moreover, the 8_2 kernel's initramfs file did not get its last
modification time updated during the downgrade, although when installing
kmod-megaraid_sas to start with, the initramfs file appeared to get
longer and have its modification time updated.

Questions:

a) Should what I did have worked ?
b) Is there a bug in the kmod-megaraid_sas package suggested ?
c) What alternative procedure should I have used ?
d) Do you need any additional info to work out what's going on ?

With thanks in advance,
Roger Sewell.