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

Jagan Teki jagan at amarulasolutions.com
Thu Apr 18 17:00:27 UTC 2019

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.
> I understand that the proper solution is to fill the gap before the vectors
> with code instead of NOPs, but I couldn't find any obvious way doing this
> in the linker script. If anyone has any idea here, I am all ears.
> Cheers,
> Andre.
> Andre Przywara (3):
>   arm: clean up asm/io.h
>   arm: introduce _relaxed MMIO accessors
>   sunxi: H6: use writel_relaxed for DRAM timing register accesses

Anyone has any further comments on this? would like to pick this before MW.


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