Re: [PATCH 1/3] x86: qemu: Describe pflash

Hi Elliot,

On 2026-07-30T19:37:45, Elliot Berman <[email protected]> wrote:
> x86: qemu: Describe pflash
>
> QEMU pflash is mapped just below 4GiB. Describe it as a cfi-flash
> node so the MTD subsystem can probe it. When a flash device is not
> exposed to the guest by QEMU, the address is still backed by ROM, so
> we'll just see no flash found rather than faulting.
>
> Signed-off-by: Elliot Berman <[email protected]>
>
> arch/x86/dts/pflash.dtsi         | 14 ++++++++++++++
>  arch/x86/dts/qemu-x86_i440fx.dts |  1 +
>  arch/x86/dts/qemu-x86_q35.dts    |  1 +
>  3 files changed, 16 insertions(+)

> diff --git a/arch/x86/dts/pflash.dtsi b/arch/x86/dts/pflash.dtsi
> new file mode 100644
> index 00000000000..ce69bcc6865
> --- /dev/null
> +++ b/arch/x86/dts/pflash.dtsi
> @@ -0,0 +1,14 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * QEMU pflash (CFI NOR) mapped just below 4GiB. QEMU aligns the pflash so
> + * that it ends at the top of the 32-bit address space.
> + * https://gitlab.com/qemu-project/qemu/-/blob/e1705a25aff35635c360bbaba4c2731d019a422a/hw/i386/pc_sysfw.c#L128
> + */

The link pins a QEMU commit hash which might go away. Please point at
a stable ref (a release tag, or just hw/i386/pc_sysfw.c on -master).

> diff --git a/arch/x86/dts/pflash.dtsi b/arch/x86/dts/pflash.dtsi
> @@ -0,0 +1,14 @@
> +/ {
> +     pflash {
> +             compatible = "cfi-flash";
> +             reg = <(0x100000000 - CONFIG_ROM_SIZE) CONFIG_ROM_SIZE>;
> +             bank-width = <1>;
> +     };
> +};

The convention would be to name it flash@<addr> - but in this case the
address varies, so what you have seems fine.

This ties the pflash window to CONFIG_ROM_SIZE, but the two aren't the
same thing - CONFIG_ROM_SIZE is what U-Boot builds itself for, whereas
QEMU's pflash1 window size is the size of the file passed to -pflash.
If a user runs QEMU with a pflash image whose size differs from
CONFIG_ROM_SIZE the reg will be wrong. Please call this out in the
comment, or better, drive it from a dedicated symbol whose name
reflects the pflash size.

Regards,
Simon