Re: [PATCH -fixes] dma-mapping: add default implementation to arch_dma_{set|clear}_uncached

[Yangyu Chen <cyy@cyyself.name>]

> On Jul 9, 2024, at 19:46, Christoph Hellwig <hch@lst.de> wrote:
> 
> On Tue, Jul 09, 2024 at 07:39:29PM +0800, Yangyu Chen wrote:
>> The reason is that some optimizations failed to apply after adding
>> some passes. I will fix the compiler later. Whatever, we should not
>> rely on this optimization to get the code being successfully compiled.
> 
> The Linux kernel relies on constant propagation and dead code
> eliminiation a lot to make code simpler and more readable.

Actually, the compiler is patched LLVM with -O2 optimization. I didn’t
turn off the optimization.

You can see what we did for the compiler here[1] and compile the
kernel with `-march=rv64imac_zicond_zicldst` added to KBUILD_CFLAGS.
I added conditional load/store pass as Intel did for the x86 APX
extension, which appeared last year (called hoist load/store in
LLVM if you want to search the PR), and then LLVM failed to optimize
this.

The only failed symbol on the kernel with `ARCH=riscv defconfig`
is `arch_dma_set_uncached` since the compiler requires all possible
values to be known. I think a pattern like in kernel/dma/direct.c:349
for symbol `arch_dma_clear_uncached`, which uses `if
(IS_ENABLE(CONFIG_ARCH_HAS_xxx)) xxx` is acceptable. But for the
symbol `arch_dma_set_uncached`, a complex analysis is needed for a
value set in a block of branches. I think we should not rely on such
compiler optimization in such a complex pattern.

In addition, patching this way can also make this symbol safer to use.

[1] https://github.com/cyyself/llvm-project/tree/zicldst-support-bugless-v3

Thanks,
Yangyu Chen