Subject: Re: [PATCH v2 6/9] powerpc/microwatt: Add support for hardware random number generator

From: Nicholas Piggin <[email-masked]>
Date: Sat, 19 Jun 2021 13:08:51 +1000

Excerpts from Paul Mackerras's message of June 18, 2021 1:47 pm:
> Microwatt's hardware RNG is accessed using the DARN instruction.
> 

I think we're getting a platforms/book3s soon with the VAS patches, 
might be a place to add the get_random_darn function.

Huh, DARN is unprivileged right? And yet we haven't wired it up in
pseries it still uses an hcall.

Anyway that's all stuff to sort out later.

Reviewed-by: Nicholas Piggin <[email-masked]>

> Signed-off-by: Paul Mackerras <[email-masked]>
> ---
>  arch/powerpc/platforms/microwatt/Kconfig  |  1 +
>  arch/powerpc/platforms/microwatt/Makefile |  2 +-
>  arch/powerpc/platforms/microwatt/rng.c    | 48 +++++++++++++++++++++++
>  3 files changed, 50 insertions(+), 1 deletion(-)
>  create mode 100644 arch/powerpc/platforms/microwatt/rng.c
> 
> 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
> 
>