Subject: Re: [PATCH v3] powerpc/powernv: Use darn instr for random_seed on p9

From: Tyrel Datwyler <[email-masked]>
Date: Fri, 4 Aug 2017 10:06:03 -0700

On 08/03/2017 06:12 PM, Matt Brown wrote:
> This adds the powernv_get_random_darn function which utilises the darn
> instruction, introduced in POWER9. The powernv_get_random_darn function
> is used as the ppc_md.get_random_seed on P9.
> 
> The DARN instruction can potentially throw an error, so we attempt to
> register the powernv_get_random_darn function up to 10 times before
> failing.
> 
> Signed-off-by: Matt Brown <[email-masked]>
> ---
> v3:
> 	- add repeat attempts to register the ppc_md.get_random_seed
> 	- fixed the PPC_DARN macro
> 	- move DARN_ERR definition
> 	- fixed commit message
> v2:
> 	- remove repeat darn attempts
> 	- move hook to rng_init
> ---
>  arch/powerpc/include/asm/ppc-opcode.h |  4 ++++
>  arch/powerpc/platforms/powernv/rng.c  | 35 ++++++++++++++++++++++++++++++++++-
>  2 files changed, 38 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/ppc-opcode.h b/arch/powerpc/include/asm/ppc-opcode.h
> index c4ced1d..aabd150 100644
> --- a/arch/powerpc/include/asm/ppc-opcode.h
> +++ b/arch/powerpc/include/asm/ppc-opcode.h
> @@ -134,6 +134,7 @@
>  #define PPC_INST_COPY			0x7c00060c
>  #define PPC_INST_COPY_FIRST		0x7c20060c
>  #define PPC_INST_CP_ABORT		0x7c00068c
> +#define PPC_INST_DARN			0x7c0005e6
>  #define PPC_INST_DCBA			0x7c0005ec
>  #define PPC_INST_DCBA_MASK		0xfc0007fe
>  #define PPC_INST_DCBAL			0x7c2005ec
> @@ -325,6 +326,9 @@
> 
>  /* Deal with instructions that older assemblers aren't aware of */
>  #define	PPC_CP_ABORT		stringify_in_c(.long PPC_INST_CP_ABORT)
> +#define PPC_DARN(t, l)		stringify_in_c(.long PPC_INST_DARN |  \
> +						___PPC_RT(t)	   |  \
> +						(((l) & 0x3) << 16))
>  #define	PPC_DCBAL(a, b)		stringify_in_c(.long PPC_INST_DCBAL | \
>  					__PPC_RA(a) | __PPC_RB(b))
>  #define	PPC_DCBZL(a, b)		stringify_in_c(.long PPC_INST_DCBZL | \
> diff --git a/arch/powerpc/platforms/powernv/rng.c b/arch/powerpc/platforms/powernv/rng.c
> index 5dcbdea..83b925c 100644
> --- a/arch/powerpc/platforms/powernv/rng.c
> +++ b/arch/powerpc/platforms/powernv/rng.c
> @@ -16,11 +16,13 @@
>  #include <linux/slab.h>
>  #include <linux/smp.h>
>  #include <asm/archrandom.h>
> +#include <asm/cputable.h>
>  #include <asm/io.h>
>  #include <asm/prom.h>
>  #include <asm/machdep.h>
>  #include <asm/smp.h>
> 
> +#define DARN_ERR 0xFFFFFFFFFFFFFFFFul
> 
>  struct powernv_rng {
>  	void __iomem *regs;
> @@ -67,6 +69,21 @@ int powernv_get_random_real_mode(unsigned long *v)
>  	return 1;
>  }
> 
> +int powernv_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;
> +}
> +
>  int powernv_get_random_long(unsigned long *v)
>  {
>  	struct powernv_rng *rng;
> @@ -135,8 +152,9 @@ static __init int rng_create(struct device_node *dn)
> 
>  static __init int rng_init(void)
>  {
> +	unsigned long darn_test;
>  	struct device_node *dn;
> -	int rc;
> +	int rc, i;
> 
>  	for_each_compatible_node(dn, NULL, "ibm,power-rng") {
>  		rc = rng_create(dn);
> @@ -150,6 +168,21 @@ static __init int rng_init(void)
>  		of_platform_device_create(dn, NULL, NULL);
>  	}
> 
> +	if (cpu_has_feature(CPU_FTR_ARCH_300)) {
> +		for (i = 0; i < 10; i++) {
> +			if (powernv_get_random_darn(&darn_test)) {
> +				ppc_md.get_random_seed =
> +					powernv_get_random_darn;
> +				break;

If you return directly here you can avoid the (i == 9) conditional every iteration of the
loop by moving the pr_warn to just outside the loop.

-Tyrel

> +			}
> +
> +			if (i == 9) {
> +				pr_warn("Failed to use powernv_get_random_darn"\
> +					"as get_random_seed");
> +			}
> +		}
> +	}
> +
>  	return 0;
>  }
>  machine_subsys_initcall(powernv, rng_init);
>