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    SubjectRe: [PATCH 1/2] arm64: mm: Do not defer reserve_crashkernel() if only ZONE_DMA32
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    On 2022/3/30 6:29, Vijay Balakrishna wrote:
    >
    >
    > On 3/24/2022 10:53 PM, Kefeng Wang wrote:
    >> The kernel could be benifit due to BLOCK_MAPPINGS, see commit
    >> 031495635b46 ("arm64: Do not defer reserve_crashkernel() for
    >> platforms with no DMA memory zones"), if there is only with
    >> ZONE_DMA32, we could set arm64_dma_phys_limit to max_zone_phys(32)
    >> earlier in arm64_memblock_init(), then we will benifit too.
    >
    > Thanks for noticing platforms with just ZONE_DMA32 config can also
    > benefit BLOCK_MAPPINGS.  I assume you have access to one where you
    > notice the difference with proposed changes and able to test.  I did
    > test proposed changes on SoC we use with IOMMU (no ZONE_DMA configs
    > enabled).
    >
    > Nits --
    > - benifit -> benefit
    > - consider making commit message clear, "then we will.." seems you are
    > referring to platforms with just ZONE_DMA32 config enabled
    > - to reflect new change consider updating comment added in commit
    > 031495635b46
    >
    One more question, could we add new cmdline(eg, dma_force_32bit) to let
    the arm64 using

    32bit dma  by default?  I think most platforms are not with small DMA
    bit(only found 20bit in Raspberry Pi 4?),

    Here is a draft(based on this patch) to do it, if the kernel boot with
    this cmdline and even ZONE_DMA

    is enabled, the kernel allows linear creation of block mapping, not sure
    this is the right way, any suggestion? Thanks.

    diff --git a/arch/arm64/include/asm/processor.h
    b/arch/arm64/include/asm/processor.h
    index 73e38d9a540c..8febed26aeed 100644
    --- a/arch/arm64/include/asm/processor.h
    +++ b/arch/arm64/include/asm/processor.h
    @@ -100,6 +100,7 @@
                                            base)
     #endif /* CONFIG_ARM64_FORCE_52BIT */

    +extern bool arm64_dma_force_32bit;
     extern phys_addr_t arm64_dma_phys_limit;
     #define ARCH_LOW_ADDRESS_LIMIT (arm64_dma_phys_limit - 1)

    diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
    index d6f64819ef89..86aab5db735b 100644
    --- a/arch/arm64/mm/init.c
    +++ b/arch/arm64/mm/init.c
    @@ -92,6 +92,16 @@ phys_addr_t __ro_after_init arm64_dma_phys_limit;
     phys_addr_t __ro_after_init arm64_dma_phys_limit = PHYS_MASK + 1;
     #endif

    +bool __ro_after_init arm64_dma_force_32bit;
    +static int __init arm64_dma_force_32bit_setup(char *p)
    +{
    +       arm64_dma_force_32bit = true;
    +       zone_dma_bits = 32;
    +
    +       return 0;
    +}
    +early_param("dma_force_32bit", arm64_dma_force_32bit_setup);
    +
     /*
      * reserve_crashkernel() - reserves memory for crash kernel
      *
    @@ -165,10 +175,11 @@ static void __init zone_sizes_init(void)
     #ifdef CONFIG_ZONE_DMA
            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);
    +       if (!arm64_dma_force_32bit) {
    +               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
    @@ -336,7 +347,7 @@ 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_DMA) || arm64_dma_force_32bit) {
                    if (IS_ENABLED(CONFIG_ZONE_DMA32))
                            arm64_dma_phys_limit = dma32_phys_limit;
                    reserve_crashkernel();
    @@ -388,7 +399,7 @@ 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 (IS_ENABLED(CONFIG_ZONE_DMA) && !arm64_dma_force_32bit)
                    reserve_crashkernel();

            memblock_dump_all();
    diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
    index 23734481318a..9431ad581927 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 (IS_ENABLED(CONFIG_ZONE_DMA) && !arm64_dma_force_32bit)
                            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 && (!IS_ENABLED(CONFIG_ZONE_DMA) ||
    arm64_dma_force_32bit)) {
                    if (crashk_res.end) {
                            __map_memblock(pgdp, crashk_res.start,
                                           crashk_res.end + 1,

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