[U-Boot] [RESEND PATCH 0/3] arm: Introduce writel/readl_relaxed accessors

[Chen-Yu Tsai]

On Mon, Apr 15, 2019 at 2:07 PM Jagan Teki <jagan at amarulasolutions.com> wrote:
>
> On Sun, Feb 10, 2019 at 9:49 PM Andre Przywara <andre.przywara at arm.com> wrote:
> >
> > Hi, this is a resend of what I posted some weeks ago, just adding the
> > missing Signed-off-by: in patch 2/3, as pointed out by Philipp. I used
> > the opportunity to add his Reviewed-by: tags on the first two patches.
> > (Many thanks for that!) The rest is unchanged.
> > -------------------
> >
> > Admittedly this is the long way round to solve some nasty SPL code size
> > problem, but it looked beneficial to others as well, so here we go:
> >
> > arch/arm/include/asm/io.h looks like it's been around since the dawn of
> > time, and was more or less blindly copied from Linux.
> > We don't use and don't need most of the definitions, and mainline Linux
> > got rid of them anyway, so patch 1/3 cleans up this header file to
> > just contain what we need in U-Boot.
> >
> > Patch 2/3 introduces readl/writel_relaxed accessors, which are cheaper,
> > but more importantly save one (barrier) instruction per accessor. This
> > helps to bring down code size, since especially DRAM controller inits in
> > SPLs tend to do a lot of MMIO.
> >
> > Consequently patch 3/3 introduces them in the Allwinner H6 DRAM driver,
> > which reduces the SPL size by a whopping 2KB, due to a twist:
> > The AArch64 exception table needs to be 2KB aligned, but we don't do
> > anything special about it the linker script. So depending on where the
> > code before the vectors ends, we have potentially large padding:
> > At the moment this last address is 0x1824 for the H6, so the vectors can
> > only start at 0x2000. By reducing the code size before the vectors by just
> > (at least) 9 instructions, the vectors start at 0x1800 and we save most of
> > the padding.
>
> How come it reduces to 2KB? I can see the diff size of 160 bytes for gcc-6.3.1
>
> ₹ aarch64-linux-gnu-size spl/u-boot-spl*
>    text       data        bss        dec        hex    filename
>   28376        408        504      29288       7268    spl/u-boot-spl
>
> ₹ aarch64-linux-gnu-size spl/u-boot-spl*
>    text       data        bss        dec        hex    filename
>   28216        408        504      29128       71c8    spl/u-boot-spl

Because of section alignment issues? I believe Andre is referring to the
size of the whole file. Since it gets loaded as a whole, the total size
is what matters, not the size of the individual sections.

ChenYu