Re: [PATCH 3/3] RISC-V: Ensure Zicbom has a valid block size

[Conor Dooley <conor@kernel.org>]

On Fri, Oct 21, 2022 at 12:59:05PM +0200, Andrew Jones wrote:
> When a DT puts zicbom in the isa string, but does not provide a block
> size, ALT_CMO_OP() will attempt to do cache operations on address
> zero since the start address will be ANDed with zero. We can't simply
> BUG() in riscv_init_cbom_blocksize() when we fail to find a block
> size because the failure will happen before logging works, leaving
> users to scratch their heads as to why the boot hung. Instead, ensure
> Zicbom is disabled and output an error which will hopefully alert
> people that the DT needs to be fixed. While at it, add a check that
> the block size is a power-of-2 too.
> 
> Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
> ---
>  arch/riscv/kernel/cpufeature.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 220be7222129..a4430a77df53 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -9,6 +9,7 @@
>  #include <linux/bitmap.h>
>  #include <linux/ctype.h>
>  #include <linux/libfdt.h>
> +#include <linux/log2.h>
>  #include <linux/module.h>
>  #include <linux/of.h>
>  #include <asm/alternative.h>
> @@ -70,6 +71,20 @@ EXPORT_SYMBOL_GPL(__riscv_isa_extension_available);
>  
>  static bool riscv_isa_extension_check(int id)
>  {
> +	switch (id) {
> +	case RISCV_ISA_EXT_ZICBOM:
> +		if (!riscv_cbom_block_size) {
> +			if (IS_ENABLED(CONFIG_RISCV_ISA_ZICBOM))

Maybe I've missed something... Why not check if !IS_ENABLED() & return
false immediately rather than doing it inside the attribute checks?
I'll be compiled out of zicbom is enabled, so not like the non-error
patch will be penalised with an extra check.

> +				pr_err("cbom-block-size not found, cannot use Zicbom\n");
> +			return false;
> +		} else if (!is_power_of_2(riscv_cbom_block_size)) {
> +			if (IS_ENABLED(CONFIG_RISCV_ISA_ZICBOM))
> +				pr_err("cbom-block-size is not a power-of-2, cannot use Zicbom\n");
> +			return false;
> +		}
> +		return true;
> +	}
> +
>  	return true;
>  }
>  
> -- 
> 2.37.3
> 
> 
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