On 14.08.2023 11:04, Geert Uytterhoeven wrote:

Hi Rafal,

On Mon, Aug 7, 2023 at 1:11 PM Rafał Miłecki [off-list ref] wrote:

quoted

On 4.08.2023 13:07, Rafał Miłecki wrote:

quoted

I triple checked that. Dropping a single unused function breaks kernel /
device stability on BCM53573!

AFAIK the only thing below diff actually affects is location of symbols
(I actually verified that by comparing System.map before and after -
over 22'000 of relocated symbols).

Can some unfortunate location of symbols cause those hangs/lockups?

I performed another experiment. First I dropped mtd_check_of_node() to
bring kernel back to the stable state.

Then I started adding useless code to the mtdchar_unlocked_ioctl(). I
ended up adding just enough to make sure all post-mtd symbols in
System.map got the same offset as in case of backporting
mtd_check_of_node().

I started experiencing lockups/hangs again.

I repeated the same test with adding dumb code to the brcm_nvram_probe()
and verifying symbols offsets following brcm_nvram_probe one.

I believe this confirms that this problem is about offset or alignment
of some specific symbol(s). The remaining question is what symbols and
how to fix or workaround that.

I had similar experiences on other ARM platforms many years ago:
bisection lead to something completely bogus, and it turned out
adding a single line of innocent code made the system lock-up or crash
unexpectedly.  It was definitely related to alignment, as adding the
right extra amount of innocent code would fix the problem. Until some
later change changing alignment again...
I never found the real cause, but the problems went away over time.
I am not sure I did enable all required errata config options, so I
may have missed some...

I already experiented some weird performance variations on Broadcom's
Northstar platform that was related to symbols layout & cache hit/miss
ratio. For that reason I use -falign-functions=32 for that whole
OpenWrt's "bcm53xx" target (it covers Northstar and BCM53573). So
this aspect should be ruled out already in my case.

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