Re: [PATCH 1/2] arm64: mm: Do not defer reserve_crashkernel() if only ZONE_DMA32

[Kefeng Wang]

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,