linux-kernel - Re: [PATCH v3 2/2] drivers: firmware: dmi_scan: Add support for reading SMBIOS from DT

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Message-ID: <CAMj1kXE6L2PTtYYqOC7+fQd7HoB_FJAOtF=p=u+AimhqkLzV8w@mail.gmail.com>
Date: Fri, 31 Oct 2025 10:05:51 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: adriana <adriana@...sta.com>
Cc: ilias.apalodimas@...aro.org, trini@...sulko.com, robh@...nel.org, 
	krzk@...nel.org, jdelvare@...e.com, frowand.list@...il.com, 
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org, vasilykh@...sta.com, 
	arm.ebbr-discuss@....com, boot-architecture@...ts.linaro.org, 
	linux-efi@...r.kernel.org, uefi-discuss@...ts.uefi.org, 
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 2/2] drivers: firmware: dmi_scan: Add support for
 reading SMBIOS from DT

Hello Adriana,

On Fri, 31 Oct 2025 at 09:41, adriana <adriana@...sta.com> wrote:
>
> From: Adriana Nicolae <adriana@...sta.com>
>
> Signed-off-by: Adriana Nicolae <adriana@...sta.com>
> ---
>  drivers/firmware/dmi_scan.c | 58 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 58 insertions(+)
>
> diff --git a/drivers/firmware/dmi_scan.c b/drivers/firmware/dmi_scan.c
> index 70d39adf50dc..acc0e18b8d0f 100644
> --- a/drivers/firmware/dmi_scan.c
> +++ b/drivers/firmware/dmi_scan.c
> @@ -10,6 +10,9 @@
>  #include <linux/random.h>
>  #include <asm/dmi.h>
>  #include <linux/unaligned.h>
> +#if IS_ENABLED(CONFIG_OF)
> +#include <linux/of.h>
> +#endif
>
>  #ifndef SMBIOS_ENTRY_POINT_SCAN_START
>  #define SMBIOS_ENTRY_POINT_SCAN_START 0xF0000
> @@ -670,6 +673,54 @@ static int __init dmi_smbios3_present(const u8 *buf)
>         return 1;
>  }
>
> +#if IS_ENABLED(CONFIG_OF)
> +/**
> + * dmi_scan_from_dt - Find and parse DMI/SMBIOS tables from the Device Tree
> + *
> + * Checks if the bootloader has passed SMBIOS table addresses via the /chosen
> + * node in the Device Tree. This follows the standard kernel DT bindings and
> + * assumes a fixed 32-byte mapping for the entry point.

Not sure what 'the standard kernel DT bindings' are, or what you mean
by 'a fixed 32-byte mapping. You could just drop this sentence, I
think, or otherwise, describe that the DT property gives us the
physical address of the SMBIOS3 entrypoint structure.

> + * Returns true if a valid table is found and successfully parsed.

if a valid entry point is found

> + */
> +static bool __init dmi_scan_from_dt(void)
> +{
> +       struct device_node *chosen;
> +       const __be64 *prop;
> +       char buf[32];
> +       void __iomem *p;
> +       bool dmi_available = false;
> +       u64 addr;
> +       int len;
> +
> +       chosen = of_find_node_by_path("/chosen");
> +       if (!chosen)
> +               return false;
> +
> +       prop = of_get_property(chosen, "linux,smbios3-table", &len);
> +       if (prop && len >= sizeof(u64)) {
> +               addr = be64_to_cpup(prop);
> +
> +               p = dmi_early_remap(addr, 32);
> +               if (p == NULL)
> +                       goto out;
> +
> +               memcpy_fromio(buf, p, sizeof(buf));
> +               dmi_early_unmap(p, 32);
> +
> +               if (!dmi_smbios3_present(buf)) {
> +                       dmi_available = true;
> +                       goto out;
> +               }
> +       }
> +
> +out:
> +       of_node_put(chosen);
> +       return dmi_available;
> +}
> +#else
> +static bool __init dmi_scan_from_dt(void) { return false; }
> +#endif /* IS_ENABLED(CONFIG_OF) */
> +
>  static void __init dmi_scan_machine(void)
>  {
>         char __iomem *p, *q;
> @@ -718,6 +769,13 @@ static void __init dmi_scan_machine(void)
>                         dmi_available = 1;
>                         return;
>                 }
> +       } else if (IS_ENABLED(CONFIG_OF) && dmi_scan_from_dt()) {

Please drop the IS_ENABLED() here, and fold it into dmi_scan_from_dt(), by doing

if (!IS_ENABLED(CONFIG_OF))
    return false;

at the beginning. This removes the need to provide two different
versions of dmi_scan_from_dt().


> +               /*
> +                * If EFI is not present or failed, try getting SMBIOS
> +                * tables from the Device Tree.
> +                */
> +               dmi_available = 1;
> +               return;
>         } else if (IS_ENABLED(CONFIG_DMI_SCAN_MACHINE_NON_EFI_FALLBACK)) {
>                 p = dmi_early_remap(SMBIOS_ENTRY_POINT_SCAN_START, 0x10000);
>                 if (p == NULL)
> --
> 2.51.0
>