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From:   Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Date:   Thu, 10 Feb 2022 17:41:28 -0800
Message-ID: <CALCv0x3uYMovfqzobM6xfr8_apZQtz=DhyM-DsJe2L_rqAj65g@mail.gmail.com>
Subject: Re: [PATCH] MIPS: ralink: mt7621: do memory detection on KSEG1
To:     Chuanhong Guo <gch981213@gmail.com>
Cc:     "open list:MIPS" <linux-mips@vger.kernel.org>,
        Rui Salvaterra <rsalvaterra@gmail.com>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi Guo,

On Thu, Feb 10, 2022 at 4:14 PM Chuanhong Guo <gch981213@gmail.com> wrote:
>
> It's reported that current memory detection code occasionally detects
> larger memory under some bootloaders.
> Current memory detection code tests whether address space wraps around
> on KSEG0, which is unreliable because it's cached.
>
> Rewrite memory size detection to perform the same test on KSEG1 instead.
> While at it, this patch also does the following two things:
> 1. use a fixed pattern instead of a random function pointer as the magic
>    value.
> 2. add an additional memory write and a second comparison as part of the
>    test to prevent possible smaller memory detection result due to
>    leftover values in memory.
>
> Fixes: 139c949f7f0a MIPS: ("ralink: mt7621: add memory detection support")
> Reported-by: Rui Salvaterra <rsalvaterra@gmail.com>
> Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
> ---
>  arch/mips/ralink/mt7621.c | 36 +++++++++++++++++++++++-------------
>  1 file changed, 23 insertions(+), 13 deletions(-)
>
> diff --git a/arch/mips/ralink/mt7621.c b/arch/mips/ralink/mt7621.c
> index d6efffd4dd20..12c8808e0dea 100644
> --- a/arch/mips/ralink/mt7621.c
> +++ b/arch/mips/ralink/mt7621.c
> @@ -22,7 +22,9 @@
>
>  #include "common.h"
>
> -static void *detect_magic __initdata = detect_memory_region;
> +#define MT7621_MEM_TEST_PATTERN         0xaa5555aa
> +
> +static u32 detect_magic __initdata;
>
>  int pcibios_root_bridge_prepare(struct pci_host_bridge *bridge)
>  {
> @@ -58,24 +60,32 @@ phys_addr_t mips_cpc_default_phys_base(void)
>         panic("Cannot detect cpc address");
>  }
>
> +static bool __init mt7621_addr_wraparound_test(phys_addr_t size)
> +{
> +       void *dm = (void *)KSEG1ADDR(&detect_magic);
> +
> +       if (CPHYSADDR(dm + size) >= MT7621_LOWMEM_MAX_SIZE)
> +               return true;
> +       __raw_writel(MT7621_MEM_TEST_PATTERN, dm);
> +       if (__raw_readl(dm) != __raw_readl(dm + size))
> +               return false;
> +       __raw_writel(!MT7621_MEM_TEST_PATTERN, dm);
> +       return __raw_readl(dm) == __raw_readl(dm + size);
> +}
> +
>  static void __init mt7621_memory_detect(void)
>  {
> -       void *dm = &detect_magic;
>         phys_addr_t size;
>
> -       for (size = 32 * SZ_1M; size < 256 * SZ_1M; size <<= 1) {
> -               if (!__builtin_memcmp(dm, dm + size, sizeof(detect_magic)))
> -                       break;
> +       for (size = 32 * SZ_1M; size <= 256 * SZ_1M; size <<= 1) {
> +               if (mt7621_addr_wraparound_test(size)) {
> +                       memblock_add(MT7621_LOWMEM_BASE, size);
> +                       return;
> +               }
>         }
>
> -       if ((size == 256 * SZ_1M) &&
> -           (CPHYSADDR(dm + size) < MT7621_LOWMEM_MAX_SIZE) &&
> -           __builtin_memcmp(dm, dm + size, sizeof(detect_magic))) {
> -               memblock_add(MT7621_LOWMEM_BASE, MT7621_LOWMEM_MAX_SIZE);
> -               memblock_add(MT7621_HIGHMEM_BASE, MT7621_HIGHMEM_SIZE);
> -       } else {
> -               memblock_add(MT7621_LOWMEM_BASE, size);
> -       }
> +       memblock_add(MT7621_LOWMEM_BASE, MT7621_LOWMEM_MAX_SIZE);
> +       memblock_add(MT7621_HIGHMEM_BASE, MT7621_HIGHMEM_SIZE);
>  }
>
>  void __init ralink_of_remap(void)
> --
> 2.34.1
>
Thanks for your change. I think this will also fix
https://lore.kernel.org/lkml/202201191557.OISJHNMi-lkp@intel.com/
since you are removing __builtin_memcmp usage.

Ilya