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From:   Jinyang He <hejinyang@loongson.cn>
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        John Crispin <john@phrozen.org>
Cc:     linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH RFC] MIPS: Remove detect_memory_region()
Date:   Wed, 24 Feb 2021 21:02:00 +0800
Message-Id: <1614171720-13221-1-git-send-email-hejinyang@loongson.cn>
Precedence: bulk

detect_memory_region() was committed by Commit 4d9f77d25268 ("MIPS: add
detect_memory_region()"). Then it was equipped by Commit dd63b00804a5
("MIPS: ralink: make use of the new memory detection code") and
Commit 9b75733b7b5e ("MIPS: ath79: make use of the new memory detection
code"). Its code is based on early ath79 platform code.

What puzzles me is that how memcmp() detect the memory region. If `break`
was touched, the function could make sense. That means memcmp() should
return zero. Otherwise, the loop will be end by size > sz_max.

I have tested detect_memory_region() on Loongson64 3A3000. On our design,
kseg0 low 256MB maps real memory and kseg0 high 256MB maps IO/PCI. The
function runs and last stopped on kseg1 where is uncached. In this process
memcmp also returned non-zero when detected kseg0 high 256MB. Then I did
another thing. memcpy first and test memcmp then (after &_end). It works
well on 3A3000 but badly on 3A4000. Maybe because kseg0 high 256MB maps
IO/PCI and it is dangerous to write like write memory.

At last, read memory from where is not memory region may always return 0.
(Or trigger exception.) This function have been used several years and
seems no error occur. Maybe it's a fallback way.

Signed-off-by: Jinyang He <hejinyang@loongson.cn>
---
 arch/mips/ath79/setup.c          |  2 +-
 arch/mips/include/asm/bootinfo.h |  2 --
 arch/mips/kernel/setup.c         | 21 ---------------------
 arch/mips/ralink/of.c            |  5 ++---
 4 files changed, 3 insertions(+), 27 deletions(-)

diff --git a/arch/mips/ath79/setup.c b/arch/mips/ath79/setup.c
index 891f495..d2a41fb 100644
--- a/arch/mips/ath79/setup.c
+++ b/arch/mips/ath79/setup.c
@@ -232,7 +232,7 @@ void __init plat_mem_setup(void)
 	ath79_detect_sys_type();
 	ath79_ddr_ctrl_init();
 
-	detect_memory_region(0, ATH79_MEM_SIZE_MIN, ATH79_MEM_SIZE_MAX);
+	memblock_add(0, ATH79_MEM_SIZE_MAX);
 
 	_machine_restart = ath79_restart;
 	_machine_halt = ath79_halt;
diff --git a/arch/mips/include/asm/bootinfo.h b/arch/mips/include/asm/bootinfo.h
index 5be10ece..0d5637e 100644
--- a/arch/mips/include/asm/bootinfo.h
+++ b/arch/mips/include/asm/bootinfo.h
@@ -90,8 +90,6 @@ const char *get_system_type(void);
 
 extern unsigned long mips_machtype;
 
-extern void detect_memory_region(phys_addr_t start, phys_addr_t sz_min,  phys_addr_t sz_max);
-
 extern void prom_init(void);
 extern void prom_free_prom_memory(void);
 extern void prom_cleanup(void);
diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
index 279be01..fbbc51e 100644
--- a/arch/mips/kernel/setup.c
+++ b/arch/mips/kernel/setup.c
@@ -86,32 +86,11 @@ static struct resource bss_resource = { .name = "Kernel bss", };
 unsigned long __kaslr_offset __ro_after_init;
 EXPORT_SYMBOL(__kaslr_offset);
 
-static void *detect_magic __initdata = detect_memory_region;
-
 #ifdef CONFIG_MIPS_AUTO_PFN_OFFSET
 unsigned long ARCH_PFN_OFFSET;
 EXPORT_SYMBOL(ARCH_PFN_OFFSET);
 #endif
 
-void __init detect_memory_region(phys_addr_t start, phys_addr_t sz_min, phys_addr_t sz_max)
-{
-	void *dm = &detect_magic;
-	phys_addr_t size;
-
-	for (size = sz_min; size < sz_max; size <<= 1) {
-		if (!memcmp(dm, dm + size, sizeof(detect_magic)))
-			break;
-	}
-
-	pr_debug("Memory: %lluMB of RAM detected at 0x%llx (min: %lluMB, max: %lluMB)\n",
-		((unsigned long long) size) / SZ_1M,
-		(unsigned long long) start,
-		((unsigned long long) sz_min) / SZ_1M,
-		((unsigned long long) sz_max) / SZ_1M);
-
-	memblock_add(start, size);
-}
-
 /*
  * Manage initrd
  */
diff --git a/arch/mips/ralink/of.c b/arch/mips/ralink/of.c
index 8286c35..2de0075 100644
--- a/arch/mips/ralink/of.c
+++ b/arch/mips/ralink/of.c
@@ -81,9 +81,8 @@ void __init plat_mem_setup(void)
 	else if (soc_info.mem_size)
 		memblock_add(soc_info.mem_base, soc_info.mem_size * SZ_1M);
 	else
-		detect_memory_region(soc_info.mem_base,
-				     soc_info.mem_size_min * SZ_1M,
-				     soc_info.mem_size_max * SZ_1M);
+		memblock_add(soc_info.mem_base,
+			     soc_info.mem_size_max * SZ_1M);
 }
 
 static int __init plat_of_setup(void)
-- 
2.1.0