[PATCH v2 09/10] ARM: p2v: switch to MOVW for Thumb2 and ARM/LPAE

[Linus Walleij]

On Mon, Sep 21, 2020 at 5:41 PM Ard Biesheuvel <ardb at kernel.org> wrote:

> In preparation for reducing the phys-to-virt minimum relative alignment
> from 16 MiB to 2 MiB, switch to patchable sequences involving MOVW
> instructions that can more easily be manipulated to carry a 12-bit
> immediate. Note that the non-LPAE ARM sequence is not updated: MOVW
> may not be supported on non-LPAE platforms, and the sequence itself
> can be updated more easily to apply the 12 bits of displacement.
>
> For Thumb2, which has many more versions of opcodes, switch to a sequence
> that can be patched by the same patching code for both versions. Note
> that the Thumb2 opcodes for MOVW and MVN are unambiguous, and have no
> rotation bits in their immediate fields, so there is no need to use
> placeholder constants in the asm blocks.
>
> While at it, drop the 'volatile' qualifiers from the asm blocks: the
> code does not have any side effects that are invisible to the compiler,
> so it is free to omit these sequences if the outputs are not used.
>
> Suggested-by: Russell King <linux at armlinux.org.uk>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>

Already the nice comments explaining what is going on
here makes the patch highly valuable. It was opaque to
me until I read the comments in this patch. Now it is
just hard to understand instead of hopeless to understand
(for my limited intellect):
Reviewed-by: Linus Walleij <linus.walleij at linaro.org>

Yours,
Linus Walleij