Subject: Re: [PATCH 07/11] powerpc: Add support for microwatt's hardware random number generator

From: Nicholas Piggin <[email-masked]>
Date: Tue, 15 Jun 2021 11:40:13 +1000

Excerpts from Paul Mackerras's message of June 15, 2021 9:02 am:
> This is accessed using the DARN instruction and should probably be
> done more generically.
> 
> Signed-off-by: Paul Mackerras <[email-masked]>
> ---
>  arch/powerpc/include/asm/archrandom.h     | 12 +++++-
>  arch/powerpc/platforms/microwatt/Kconfig  |  1 +
>  arch/powerpc/platforms/microwatt/Makefile |  2 +-
>  arch/powerpc/platforms/microwatt/rng.c    | 48 +++++++++++++++++++++++
>  4 files changed, 61 insertions(+), 2 deletions(-)
>  create mode 100644 arch/powerpc/platforms/microwatt/rng.c
> 
> diff --git a/arch/powerpc/include/asm/archrandom.h b/arch/powerpc/include/asm/archrandom.h
> index 9a53e29680f4..e8ae0f7740f9 100644
> --- a/arch/powerpc/include/asm/archrandom.h
> +++ b/arch/powerpc/include/asm/archrandom.h
> @@ -8,12 +8,22 @@
>  
>  static inline bool __must_check arch_get_random_long(unsigned long *v)
>  {
> +	if (ppc_md.get_random_seed)
> +		return ppc_md.get_random_seed(v);
> +
>  	return false;
>  }
>  
>  static inline bool __must_check arch_get_random_int(unsigned int *v)
>  {
> -	return false;
> +	unsigned long val;
> +	bool rc;
> +
> +	rc = arch_get_random_long(&val);
> +	if (rc)
> +		*v = val;
> +
> +	return rc;
>  }
>  

I would be happier if you didn't change this (or at least put it in its 
own patch explaining why it's not going to slow down other platforms).

I'm assuming the main problem you have is seeding the rngs at boot? It
should be enough to have ppc_md.get_random_seed for that.

(BTW I wonder should lib/random32.c be changed to call 
arch_get_random_seed_long() for seeding)

>  static inline bool __must_check arch_get_random_seed_long(unsigned long *v)
> diff --git a/arch/powerpc/platforms/microwatt/Kconfig b/arch/powerpc/platforms/microwatt/Kconfig
> index 50ed0cedb5f1..8f6a81978461 100644
> --- a/arch/powerpc/platforms/microwatt/Kconfig
> +++ b/arch/powerpc/platforms/microwatt/Kconfig
> @@ -7,6 +7,7 @@ config PPC_MICROWATT
>  	select PPC_ICP_NATIVE
>  	select PPC_NATIVE
>  	select PPC_UDBG_16550
> +	select ARCH_RANDOM
>  	help
>            This option enables support for FPGA-based Microwatt implementations.
>  
> diff --git a/arch/powerpc/platforms/microwatt/Makefile b/arch/powerpc/platforms/microwatt/Makefile
> index e6885b3b2ee7..116d6d3ad3f0 100644
> --- a/arch/powerpc/platforms/microwatt/Makefile
> +++ b/arch/powerpc/platforms/microwatt/Makefile
> @@ -1 +1 @@
> -obj-y	+= setup.o
> +obj-y	+= setup.o rng.o
> diff --git a/arch/powerpc/platforms/microwatt/rng.c b/arch/powerpc/platforms/microwatt/rng.c
> new file mode 100644
> index 000000000000..3d8ee6eb7dad
> --- /dev/null
> +++ b/arch/powerpc/platforms/microwatt/rng.c
> @@ -0,0 +1,48 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Derived from arch/powerpc/platforms/powernv/rng.c, which is:
> + * Copyright 2013, Michael Ellerman, IBM Corporation.
> + */
> +
> +#define pr_fmt(fmt)	"microwatt-rng: " fmt
> +
> +#include <linux/kernel.h>
> +#include <linux/smp.h>
> +#include <asm/archrandom.h>
> +#include <asm/cputable.h>
> +#include <asm/machdep.h>
> +
> +#define DARN_ERR 0xFFFFFFFFFFFFFFFFul
> +
> +int microwatt_get_random_darn(unsigned long *v)
> +{
> +	unsigned long val;
> +
> +	/* Using DARN with L=1 - 64-bit conditioned random number */
> +	asm volatile(PPC_DARN(%0, 1) : "=r"(val));
> +
> +	if (val == DARN_ERR)
> +		return 0;
> +
> +	*v = val;
> +
> +	return 1;
> +}
> +
> +static __init int rng_init(void)
> +{
> +	unsigned long val;
> +	int i;
> +
> +	for (i = 0; i < 10; i++) {
> +		if (microwatt_get_random_darn(&val)) {
> +			ppc_md.get_random_seed = microwatt_get_random_darn;
> +			return 0;
> +		}
> +	}
> +
> +	pr_warn("Unable to use DARN for get_random_seed()\n");
> +
> +	return -EIO;
> +}
> +machine_subsys_initcall(, rng_init);
> -- 
> 2.31.1
> 
>