linux-kernel - Re: [PATCH] staging: vchiq: add 'static' to function definition

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Date:   Sun, 30 Oct 2022 13:43:46 +1100
From:   "Scott J. Crouch" <scottjcrouch@...il.com>
To:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Broadcom internal kernel review list 
        <bcm-kernel-feedback-list@...adcom.com>,
        kernel-janitors@...r.kernel.org,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: vchiq: add 'static' to function definition

On 28/10/22 01:08, Arnd Bergmann wrote:
> On Sat, Oct 22, 2022, at 10:07, Greg Kroah-Hartman wrote:
>> On Sat, Oct 22, 2022 at 03:35:48PM +1100, Scott J. Crouch wrote:
> 
>> Nice try, but this breaks the build in a very horrible and strange way
>> that no one has been able to figure out yet:
> 
> I got curious and figured out what happens:
> 
> Without CONFIG_OF, of_match_node() always returns NULL, so
> vchiq_probe() returns -EINVAL unconditionally before calling
> vchiq_platform_init(). 
> 
> If vchiq_platform_init() is marked 'static', gcc's dead code
> elimination then eliminates it, which in turn means that
> 'g_fragments_base' is never initialized and gets replaced
> with a NULL pointer.

Good spotting.  Actually, I was clumsily learning how to run sparse on the
staging directory and wasn't sure what config to use that didn't involve
enabling everything manually.  But from what I can tell, it's OF_OVERLAY (or
something downstream of that) that gets rid of the warning.  BCM2835_VCHIQ
requires BCM_VIDEOCORE which already depends on OF.  I'm confused because I
don't actually know how to reproduce the gcc warning without manually enabling
the module such that OF is enabled but OF_OVERLAY isn't, since allmodconfig
enables both -- maybe there's a way to do that.

I'm also confused why devm_rpi_firmware_get() doesn't have the same problem as
of_match_node() just above it -- it returns NULL when RASPBERRYPI_FIRMWARE is
unset, but gcc still builds without the warning.

Scott.