linux-kernel - Re: [PATCH] arm64: add early fixmap initialization flag

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Date: Mon, 19 Feb 2024 10:48:26 +0000
From: Mark Rutland <mark.rutland@....com>
To: skseofh@...il.com
Cc: catalin.marinas@....com, will@...nel.org, ryan.roberts@....com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: add early fixmap initialization flag

On Sat, Feb 17, 2024 at 11:03:26PM +0900, skseofh@...il.com wrote:
> From: Daero Lee <skseofh@...il.com>
> 
> early_fixmap_init may be called multiple times. Since there is no
> change in the page table after early fixmap initialization, an
> initialization flag was added.

Why is that better?

We call early_fixmap_init() in two places:

* early_fdt_map()
* setup_arch()

.. and to get to setup_arch() we *must* have gone through early_fdt_map(),
since __primary_switched() calls that before going to setup_arch().

So AFAICT we can remove the second call to early_fixmap_init() in setup_arch(),
and rely on the earlier one in early_fdt_map().

Mark.

> 
> Signed-off-by: Daero Lee <skseofh@...il.com>
> ---
>  arch/arm64/mm/fixmap.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
> index c0a3301203bd..fbdd5f30f3a1 100644
> --- a/arch/arm64/mm/fixmap.c
> +++ b/arch/arm64/mm/fixmap.c
> @@ -32,6 +32,8 @@ static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __page_aligned_bss;
>  static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
>  static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
>  
> +static int early_fixmap_initialized __initdata;
> +
>  static inline pte_t *fixmap_pte(unsigned long addr)
>  {
>  	return &bm_pte[BM_PTE_TABLE_IDX(addr)][pte_index(addr)];
> @@ -100,10 +102,15 @@ void __init early_fixmap_init(void)
>  	unsigned long addr = FIXADDR_TOT_START;
>  	unsigned long end = FIXADDR_TOP;
>  
> +	if (early_fixmap_initialized)
> +		return;
> +
>  	pgd_t *pgdp = pgd_offset_k(addr);
>  	p4d_t *p4dp = p4d_offset(pgdp, addr);
>  
>  	early_fixmap_init_pud(p4dp, addr, end);
> +
> +	early_fixmap_initialized = 1;
>  }
>  
>  /*
> -- 
> 2.25.1
>