Re: [PATCH -next] fbdev: offb: allow build when DRM_OFDRM=m

On Wed, Nov 23, 2022, at 04:16, Randy Dunlap wrote:

Fix build when CONFIG_FB_OF=y and CONFIG_DRM_OFDRM=m.
When the latter symbol is =m, kconfig downgrades (limits) the 'select's
under FB_OF to modular (=m). This causes undefined symbol references:

powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x58): 
undefined reference to `cfb_fillrect'
powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x60): 
undefined reference to `cfb_copyarea'
powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x68): 
undefined reference to `cfb_imageblit'

Fix this by allowing FB_OF any time that DRM_OFDRM != y so that the
selected FB_CFB_* symbols will become =y instead of =m.

In tristate logic (for DRM_OFDRM), this changes the dependency from
    !DRM_OFDRM	== 2 - 1 == 1 => modular only (or disabled)
to (boolean)
    DRM_OFDRM != y == y, allowing the 'select's to cause the
FB_CFB_* symbols to =y instead of =m.

Is it actually a useful configuration to have OFDRM=m and
FB_OF=y though? I would expect in that case that the OFDRM
driver never binds to a device because it's already owned
by FB_OF.

I would instead make this 'depends on DRM_OFDRM=n', which
completely eliminates configs that have both driver enabled.

A nicer change would be to make FB_OF a tristate symbol,
which makes it possible to load one of the two modules if
both are enabled =m, while only allowing one of them to
be =y if the other is completely disabled. It looks like
offb was originally written to be usable as a loadable module,
but Kconfig has prevented this since at least the start of
the git history.

     Arnd