linux-kernel - Re: [PATCH v5] x86/microcode: move away from using a fake platform device

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Message-ID: <6bf68fb1-32c2-4beb-b012-149947dd2002@intel.com>
Date: Tue, 1 Jul 2025 10:37:28 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-kernel@...r.kernel.org>
CC: Borislav Petkov <bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
	<x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v5] x86/microcode: move away from using a fake platform
 device

On 7/1/2025 3:54 AM, Greg Kroah-Hartman wrote:
> Downloading firmware needs a device to hang off of, and so a platform
> device seemed like the simplest way to do this.  Now that we have a faux
> device interface, use that instead as this "microcode device" is not
> anything resembling a platform device at all.
> 
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: <x86@...nel.org>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
> v5: - rebase against 6.16-rc4 and actually cc: the relevant maintainers
>       this time, doh!
> v4: - api tweaked due to parent pointer added to faux_device create
>       function.
> v3: - no change
> v2: - new patch in a larger series
> 
>  arch/x86/kernel/cpu/microcode/core.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
> index fe50eb5b7c4a..b92e09a87c69 100644
> --- a/arch/x86/kernel/cpu/microcode/core.c
> +++ b/arch/x86/kernel/cpu/microcode/core.c
> @@ -17,8 +17,8 @@
>  
>  #define pr_fmt(fmt) "microcode: " fmt
>  
> -#include <linux/platform_device.h>
>  #include <linux/stop_machine.h>
> +#include <linux/device/faux.h>
>  #include <linux/syscore_ops.h>
>  #include <linux/miscdevice.h>
>  #include <linux/capability.h>
> @@ -249,7 +249,7 @@ static void reload_early_microcode(unsigned int cpu)
>  }
>  
>  /* fake device for request_firmware */
> -static struct platform_device	*microcode_pdev;
> +static struct faux_device *microcode_fdev;
>  
>  #ifdef CONFIG_MICROCODE_LATE_LOADING
>  /*
> @@ -690,7 +690,7 @@ static int load_late_locked(void)
>  	if (!setup_cpus())
>  		return -EBUSY;
>  
> -	switch (microcode_ops->request_microcode_fw(0, &microcode_pdev->dev)) {
> +	switch (microcode_ops->request_microcode_fw(0, &microcode_fdev->dev)) {
>  	case UCODE_NEW:
>  		return load_late_stop_cpus(false);
>  	case UCODE_NEW_SAFE:
> @@ -841,9 +841,9 @@ static int __init microcode_init(void)
>  	if (early_data.new_rev)
>  		pr_info_once("Updated early from: 0x%08x\n", early_data.old_rev);
>  
> -	microcode_pdev = platform_device_register_simple("microcode", -1, NULL, 0);
> -	if (IS_ERR(microcode_pdev))
> -		return PTR_ERR(microcode_pdev);
> +	microcode_fdev = faux_device_create("microcode", NULL, NULL);
> +	if (!microcode_fdev)
> +		return -ENODEV;
>  

I was going to suggest including an error print here, since
microcode_init() is quite verbose otherwise. But it looks like
faux_device_create() itself would print a message in most cases if it
can't create the device. Anyway, this isn't directly related to proposed
change.

Reviewed-by: Sohil Mehta <sohil.mehta@...el.com>

>  	dev_root = bus_get_dev_root(&cpu_subsys);
>  	if (dev_root) {
> @@ -862,7 +862,7 @@ static int __init microcode_init(void)
>  	return 0;
>  
>   out_pdev:
> -	platform_device_unregister(microcode_pdev);
> +	faux_device_destroy(microcode_fdev);
>  	return error;
>  
>  }