[PATCH v2 1/3] arm64: Mention 4K aligned load addresses in the PIE Kconfig help

[Edgar E. Iglesias <edgar.iglesias@xilinx.com>]

On Sun, Sep 06, 2020 at 11:16:17PM +0100, Andr? Przywara wrote:
> On 04/09/2020 19:42, Stephen Warren wrote:
> > On 9/4/20 3:07 AM, Edgar E. Iglesias wrote:
> >> From: "Edgar E. Iglesias" <edgar.iglesias@xilinx.com>
> >>
> >> Mention the requirement of 4K aligned load addresses in the
> >> help section for the POSITION_INDEPENDENT option.
> >>
> >> Suggested-by: Michal Simek <michal.simek@xilinx.com>
> >> Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
> >> ---
> >>  arch/arm/Kconfig | 3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> >> index f30c2639ec..c144c08612 100644
> >> --- a/arch/arm/Kconfig
> >> +++ b/arch/arm/Kconfig
> >> @@ -21,6 +21,9 @@ config POSITION_INDEPENDENT
> >>  	  information that is embedded in the binary to support U-Boot
> >>  	  relocating itself to the top-of-RAM later during execution.
> >>  
> >> +	  When this option is enabled, U-Boot needs to be loaded at a
> >> +	  4K aligned address.
> > 
> > I don't believe this restriction should be documented as part of
> > POSITION_INDEPENDENT; the restriction always exists at least for 64-bit
> > ARM, since arch/arm/lib/relocate_64.S relocate_code uses the same
> > assembly sequence that imposes this restriction, and IIUC that code is
> > unconditionally used.
> 
> While this is true, the difference is that without POSITION_INDEPENDENT
> the alignment is easily determined by the hardcoded load address. So we
> should actually have a build time check on this.
> 
> With POSITION_INDEPENDENT, however, the load address is only known at
> runtime (somewhat under the user's control, if you like). So a warning
> or hint here might be useful. But maybe it should be noted as a general
> restriction in the paragraph above:
> " ... from almost any address" => "from almost any 4K aligned address"
>

That sounds good to me.

Thanks,
Edgar