Re: RFC Need advice on reworking gpio-ep93xx.c to DT support

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

On Mon, Mar 22, 2021 at 11:33 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Mon, Mar 22, 2021 at 6:00 PM Alexander Sverdlin
> <alexander.sverdlin@xxxxxxxxx> wrote:
> > On Mon, 2021-03-22 at 19:48 +0300, nikita.shubin@xxxxxxxxxxx wrote:
> > > > > >  Note that the GPIO banks are registered a bit goofy, Ports C and F are
> > > > > >  not in order. They have been that way since the original Cirrus "crater"
> > > > > >  code base. If I remember correctly this was somewhere back in the 2.6.x
> > > > > >  kernel. Please make sure the GPIO numbers stay the same so that any
> > > > > >  userspace code does not break.
> > > >
> > > > >  I'm sceptical about this DT convertion.
> > > >
> > > > I'm in the same boat. One of the reasons I have not tried to convert it...
> > >
> > > I find this a bit confusing, so you think ep93xx shouldn't be touched at all ?
> > >
> > > AFAIK the question is reworking to DT or it will be dropped eventually:
> > >
> > > https://lore.kernel.org/lkml/CAK8P3a2VW8T+yYUG1pn1yR-5eU4jJXe1+M_ot6DAvfr2KyXCzQ@xxxxxxxxxxxxxx/
> >
> > I somehow missed the Jan Email even though I should be in the "maintainers"
> > for EP93xx. I still know about thousands of devices running 24/7 with mainline Linux.
> >
> > Is it really about "DT conversion or die"?
> > These systems really have very tight RAM and Flash budgets...
>
> I would very much like to see the platform get modernized, though as far
> as I'm concerned, the DT conversion is not the highest priority here.
>
> One thing I really want to see happen is to move the few remaining
> private implementations of the clk API over to the common-clk framework,
> and once that is done, allow ep93xx to be built into the same kernel
> as all other arm9 based platforms. There are still a couple of other platforms
> that are missing a little work for this, but it should be doable.
>
> Unfortunately, building a multiplatform kernel makes the kernel image
> somewhat larger because it includes the code for CONFIG_OF, though
> it does not have the runtime overhead for the DT data structures that you
> get when running a DT-enabled kernel. Enabling CONFIG_USE_OF
> increased the ep93xx_defconfig build for me by 128KB, replacing
> the private clk driver with CONFIG_COMMON_CLOCK (and no driver)
> on top added another 50KB, and finally enabling multiplatform added
> another 2KB. In total, that is 2.7% total bloat in just the kernel image:
>
>    text    data     bss     dec     hex filename
> 5677321 1119704   90556 6887581 69189d build/tmp/vmlinux
> 5782854 1143720   92188 7018762 6b190a build/tmp/vmlinux-use_of
> 5830020 1153408   89396 7072824 6bec38 build/tmp/vmlinux-of+clk
> 5829320 1153920   91308 7074548 6bf2f4 build/tmp/vmlinux-multi

Hence you'll get about the same kernel image size increase either
by converting to DT and COMMON_CLOCK now, or by doing nothing and
booting v5.16-rc1 in six months...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds