linux-kernel - Re: [PATCH] LoongArch: Support mem=SIZE kernel parameter

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Message-ID: <0561da2c-a4d7-49bb-8394-930f10880610@linux.dev>
Date: Wed, 2 Jul 2025 10:11:48 +0800
From: Yanteng Si <si.yanteng@...ux.dev>
To: Ming Wang <wangming01@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>,
 WANG Xuerui <kernel@...0n.name>, Andrew Morton <akpm@...ux-foundation.org>,
 Bibo Mao <maobibo@...ngson.cn>, Hari Bathini <hbathini@...ux.ibm.com>,
 Guo Weikang <guoweikang.kernel@...il.com>,
 Sourabh Jain <sourabhjain@...ux.ibm.com>, Usama Arif
 <usamaarif642@...il.com>, loongarch@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Cc: lixuefeng@...ngson.cn, chenhuacai@...ngson.cn, gaojuxin@...ngson.cn
Subject: Re: [PATCH] LoongArch: Support mem=SIZE kernel parameter

在 7/1/25 5:04 PM, Ming Wang 写道:
> The LoongArch mem= parameter parser was previously limited to the
> mem=SIZE@...RT format. This was inconvenient for the common use case
> of simply capping the total system memory, as it forced users to
> manually specify a start address. It was also inconsistent with the
> behavior on other architectures.
> 
> This patch enhances the parser in early_parse_mem() to also support the
> more user-friendly mem=SIZE format. The implementation now checks for
> the presence of the '@' symbol to determine the user's intent:
> 
> - If mem=SIZE is provided (no '@'), the kernel now calls
>    memblock_enforce_memory_limit(). This trims memory from the top down
>    to the specified size.
> - If mem=SIZE@...RT is used, the original behavior is retained for
>    backward compatibility. This allows for defining specific memory
>    banks.
> 
> This change introduces an important usage rule reflected in the code's
> comments: the mem=SIZE format should only be specified once on the
> kernel command line. It acts as a single, global cap on total memory. In
> contrast, the mem=SIZE@...RT format can be used multiple times to
> define several distinct memory regions.
> 
> Signed-off-by: Ming Wang <wangming01@...ngson.cn>
> ---
>   arch/loongarch/kernel/setup.c | 18 ++++++++++--------
>   1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/loongarch/kernel/setup.c b/arch/loongarch/kernel/setup.c
> index b99fbb388fe0..af59ba180dc2 100644
> --- a/arch/loongarch/kernel/setup.c
> +++ b/arch/loongarch/kernel/setup.c
> @@ -191,6 +191,16 @@ static int __init early_parse_mem(char *p)
>   		return -EINVAL;
>   	}
>   
> +	start = 0;
> +	size = memparse(p, &p);
> +	if (*p == '@')	/* Every mem=... should contain '@' */
> +		start = memparse(p + 1, &p);
> +	else {			/* Only one mem=... is allowed if no '@' */
> +		usermem = 1;
> +		memblock_enforce_memory_limit(size);
> +		return 0;
> +	}
> +
>   	/*
>   	 * If a user specifies memory size, we
>   	 * blow away any automatically generated
> @@ -201,14 +211,6 @@ static int __init early_parse_mem(char *p)
>   		memblock_remove(memblock_start_of_DRAM(),
>   			memblock_end_of_DRAM() - memblock_start_of_DRAM());
>   	}
> -	start = 0;
> -	size = memparse(p, &p);
> -	if (*p == '@')
> -		start = memparse(p + 1, &p);
> -	else {
> -		pr_err("Invalid format!\n");
> -		return -EINVAL;
> -	}
I don't understand. Isn't it better to modify the else{} directly here?

Thanks,
Yanteng