Re: [PATCH v2 resend 2/3] arm64: mm: Don't defer reserve_crashkernel() with dma_force_32bit

[Vijay Balakrishna]

On 3/31/2022 12:40 AM, Kefeng Wang wrote:
> ARM64 enable ZONE_DMA by default, and with ZONE_DMA crash kernel
> memory reservation is delayed until DMA zone memory range size
> initilazation performed in zone_sizes_init(), but for most platforms
> use 32bit dma_zone_bits, so add dma_force_32bit kernel parameter
> if ZONE_DMA enabled, and initialize arm64_dma_phys_limit to
> dma32_phys_limit in arm64_memblock_init() if dma_force_32bit
> is setup, this could let the crash kernel reservation earlier,
> and allows linear creation with block mapping.
>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>

I don't see any problem with the approach.  Hope you or someone can test 
to make sure no surprises on RPi4 with the proposed change.  I do 
understand on RPi4 --

- both ZONE_DMA and ZONE_DMA32 are enabled
- one wouldn't use dma_force_32bit kernel parameter
- crashkernel_could_early_reserve() would return false to preserve late 
reserve of crash kernel memory

nit --
- consider renaming crashkernel_could_early_reserve() => 
crashkernel_early_reserve()

Reviewed-by: Vijay Balakrishna <vijayb@xxxxxxxxxxxxxxxxxxx>

> ---
>   arch/arm64/include/asm/kexec.h |  1 +
>   arch/arm64/mm/init.c           | 42 ++++++++++++++++++++++++++--------
>   arch/arm64/mm/mmu.c            |  4 ++--
>   3 files changed, 36 insertions(+), 11 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kexec.h b/arch/arm64/include/asm/kexec.h
> index 9839bfc163d7..8bea40aea359 100644
> --- a/arch/arm64/include/asm/kexec.h
> +++ b/arch/arm64/include/asm/kexec.h
> @@ -95,6 +95,7 @@ void cpu_soft_restart(unsigned long el2_switch, unsigned long entry,
>   		      unsigned long arg0, unsigned long arg1,
>   		      unsigned long arg2);
>   #endif
> +bool crashkernel_could_early_reserve(void);
>   
>   #define ARCH_HAS_KIMAGE_ARCH
>   
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index fb01eb489fa9..0aafa9181607 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -66,7 +66,8 @@ EXPORT_SYMBOL(memstart_addr);
>    * depending on DMA memory zones configs (ZONE_DMA) --
>    *
>    * In absence of ZONE_DMA and ZONE_DMA32 configs arm64_dma_phys_limit
> - * initialized here and if only with ZONE_DMA32 arm64_dma_phys_limit
> + * initialized here, and if only with ZONE_DMA32 or if with ZONE_DMA
> + * and dma_force_32bit kernel parameter, the arm64_dma_phys_limit is
>    * initialised to dma32_phys_limit. This lets early reservation of
>    * crash kernel memory which has a dependency on arm64_dma_phys_limit.
>    * Reserving memory early for crash kernel allows linear creation of block
> @@ -92,6 +93,27 @@ phys_addr_t __ro_after_init arm64_dma_phys_limit;
>   phys_addr_t __ro_after_init arm64_dma_phys_limit = PHYS_MASK + 1;
>   #endif
>   
> +static bool __ro_after_init arm64_dma_force_32bit;
> +#ifdef CONFIG_ZONE_DMA
> +static int __init arm64_dma_force_32bit_setup(char *p)
> +{
> +	zone_dma_bits = 32;
> +	arm64_dma_force_32bit = true;
> +
> +	return 0;
> +}
> +early_param("dma_force_32bit", arm64_dma_force_32bit_setup);
> +#endif
> +
> +bool __init crashkernel_could_early_reserve(void)
> +{
> +	if (!IS_ENABLED(CONFIG_ZONE_DMA))
> +		return true;
> +	if (arm64_dma_force_32bit)
> +		return true;
> +	return false;
> +}
> +
>   /*
>    * reserve_crashkernel() - reserves memory for crash kernel
>    *
> @@ -163,12 +185,14 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max)
>   {
>   	unsigned long max_zone_pfns[MAX_NR_ZONES]  = {0};
>   #ifdef CONFIG_ZONE_DMA
> -	unsigned int acpi_zone_dma_bits;
> -	unsigned int dt_zone_dma_bits;
> +	if (!arm64_dma_force_32bit) {
> +		unsigned int acpi_zone_dma_bits;
> +		unsigned int dt_zone_dma_bits;
>   
> -	acpi_zone_dma_bits = fls64(acpi_iort_dma_get_max_cpu_address());
> -	dt_zone_dma_bits = fls64(of_dma_get_max_cpu_address(NULL));
> -	zone_dma_bits = min3(32U, dt_zone_dma_bits, acpi_zone_dma_bits);
> +		acpi_zone_dma_bits = fls64(acpi_iort_dma_get_max_cpu_address());
> +		dt_zone_dma_bits = fls64(of_dma_get_max_cpu_address(NULL));
> +		zone_dma_bits = min3(32U, dt_zone_dma_bits, acpi_zone_dma_bits);
> +	}
>   	arm64_dma_phys_limit = max_zone_phys(zone_dma_bits);
>   	max_zone_pfns[ZONE_DMA] = PFN_DOWN(arm64_dma_phys_limit);
>   #endif
> @@ -336,8 +360,8 @@ void __init arm64_memblock_init(void)
>   	early_init_fdt_scan_reserved_mem();
>   
>   	dma32_phys_limit = max_zone_phys(32);
> -	if (!IS_ENABLED(CONFIG_ZONE_DMA)) {
> -		if (IS_ENABLED(CONFIG_ZONE_DMA32))
> +	if (crashkernel_could_early_reserve()) {
> +		if (IS_ENABLED(CONFIG_ZONE_DMA32) || arm64_dma_force_32bit)
>   			arm64_dma_phys_limit = dma32_phys_limit;
>   		reserve_crashkernel();
>   	}
> @@ -388,7 +412,7 @@ void __init bootmem_init(void)
>   	 * request_standard_resources() depends on crashkernel's memory being
>   	 * reserved, so do it here.
>   	 */
> -	if (IS_ENABLED(CONFIG_ZONE_DMA))
> +	if (!crashkernel_could_early_reserve())
>   		reserve_crashkernel();
>   
>   	memblock_dump_all();
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 23734481318a..8f7e8452d906 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -529,7 +529,7 @@ static void __init map_mem(pgd_t *pgdp)
>   
>   #ifdef CONFIG_KEXEC_CORE
>   	if (crash_mem_map) {
> -		if (IS_ENABLED(CONFIG_ZONE_DMA))
> +		if (!crashkernel_could_early_reserve())
>   			flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>   		else if (crashk_res.end)
>   			memblock_mark_nomap(crashk_res.start,
> @@ -570,7 +570,7 @@ static void __init map_mem(pgd_t *pgdp)
>   	 * through /sys/kernel/kexec_crash_size interface.
>   	 */
>   #ifdef CONFIG_KEXEC_CORE
> -	if (crash_mem_map && !IS_ENABLED(CONFIG_ZONE_DMA)) {
> +	if (crash_mem_map && crashkernel_could_early_reserve()) {
>   		if (crashk_res.end) {
>   			__map_memblock(pgdp, crashk_res.start,
>   				       crashk_res.end + 1,