Subject: Re: [PATCH v5 3/3] hwrng: add mtk-sec-rng driver

From: Neal Liu <[email-masked]>
Date: Thu, 28 Nov 2019 23:02:55 +0800

On Wed, 2019-11-27 at 23:03 +0800, Ard Biesheuvel wrote:
> On Wed, 27 Nov 2019 at 15:23, Neal Liu <[email-masked]> wrote:
> >
> > For MediaTek SoCs on ARMv8 with TrustZone enabled, peripherals like
> > entropy sources is not accessible from normal world (linux) and
> > rather accessible from secure world (ATF/TEE) only. This driver aims
> > to provide a generic interface to ATF rng service.
> >
> > Signed-off-by: Neal Liu <[email-masked]>
> > ---
> >  drivers/char/hw_random/Kconfig       |   16 ++++++
> >  drivers/char/hw_random/Makefile      |    1 +
> >  drivers/char/hw_random/mtk-sec-rng.c |  103 ++++++++++++++++++++++++++++++++++
> >  3 files changed, 120 insertions(+)
> >  create mode 100644 drivers/char/hw_random/mtk-sec-rng.c
> >
> > diff --git a/drivers/char/hw_random/Kconfig b/drivers/char/hw_random/Kconfig
> > index 25a7d8f..f08c852 100644
> > --- a/drivers/char/hw_random/Kconfig
> > +++ b/drivers/char/hw_random/Kconfig
> > @@ -398,6 +398,22 @@ config HW_RANDOM_MTK
> >
> >           If unsure, say Y.
> >
> > +config HW_RANDOM_MTK_SEC
> > +       tristate "MediaTek Security Random Number Generator support"
> > +       depends on HW_RANDOM
> > +       depends on ARCH_MEDIATEK || COMPILE_TEST
> > +       default HW_RANDOM
> > +         help
> > +         This driver provides kernel-side support for the Random Number
> > +         Generator hardware found on MediaTek SoCs. The difference with
> > +         mtk-rng is the Random Number Generator hardware is secure
> > +         access only.
> > +
> > +         To compile this driver as a module, choose M here. the
> > +         module will be called mtk-sec-rng.
> > +
> > +         If unsure, say Y.
> > +
> >  config HW_RANDOM_S390
> >         tristate "S390 True Random Number Generator support"
> >         depends on S390
> > diff --git a/drivers/char/hw_random/Makefile b/drivers/char/hw_random/Makefile
> > index 7c9ef4a..bee5412 100644
> > --- a/drivers/char/hw_random/Makefile
> > +++ b/drivers/char/hw_random/Makefile
> > @@ -36,6 +36,7 @@ obj-$(CONFIG_HW_RANDOM_PIC32) += pic32-rng.o
> >  obj-$(CONFIG_HW_RANDOM_MESON) += meson-rng.o
> >  obj-$(CONFIG_HW_RANDOM_CAVIUM) += cavium-rng.o cavium-rng-vf.o
> >  obj-$(CONFIG_HW_RANDOM_MTK)    += mtk-rng.o
> > +obj-$(CONFIG_HW_RANDOM_MTK_SEC)        += mtk-sec-rng.o
> >  obj-$(CONFIG_HW_RANDOM_S390) += s390-trng.o
> >  obj-$(CONFIG_HW_RANDOM_KEYSTONE) += ks-sa-rng.o
> >  obj-$(CONFIG_HW_RANDOM_OPTEE) += optee-rng.o
> > diff --git a/drivers/char/hw_random/mtk-sec-rng.c b/drivers/char/hw_random/mtk-sec-rng.c
> > new file mode 100644
> > index 0000000..69ddeca
> > --- /dev/null
> > +++ b/drivers/char/hw_random/mtk-sec-rng.c
> > @@ -0,0 +1,103 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Copyright (C) 2019 MediaTek Inc.
> > + */
> > +
> > +#include <linux/arm-smccc.h>
> > +#include <linux/hw_random.h>
> > +#include <linux/module.h>
> > +#include <linux/of.h>
> > +#include <linux/platform_device.h>
> > +#include <linux/soc/mediatek/mtk_sip_svc.h>
> > +
> > +#define MTK_SEC_RNG_MAGIC      0x74726e67
> > +#define SMC_RET_NUM            4
> > +#define MTK_SEC_RND_SIZE       (sizeof(u32) * SMC_RET_NUM)
> > +
> > +static void mtk_sec_get_rnd(uint32_t *val)
> > +{
> > +       struct arm_smccc_res res;
> > +
> > +       arm_smccc_smc(MTK_SIP_KERNEL_GET_RND,
> > +                     MTK_SEC_RNG_MAGIC, 0, 0, 0, 0, 0, 0, &res);
> > +
> 
> Can this call never fail? How does the firmware signal that something
> is wrong with the underlying hardware?
> 

The smc call is supported in both ARMv7 & ARMv8 architectures.But yes,
it should check hardware status before assigning it.

We would like to check that if hardware is something wrong, all return
value will be zero. ex:

	if (!res.a0 && !res.a1 && !res.a2 && !res.a3)
		return false;

> > +       val[0] = res.a0;
> > +       val[1] = res.a1;
> > +       val[2] = res.a2;
> > +       val[3] = res.a3;

	return true;
> > +}
> > +
> > +static int mtk_sec_rng_read(struct hwrng *rng, void *buf, size_t max, bool wait)
> > +{
> > +       u32 val[4] = {0};
> > +       int retval = 0;
> > +       int i;
> > +
> > +       while (max >= MTK_SEC_RND_SIZE) {
> > +               mtk_sec_get_rnd(val);
> > +
> > +               for (i = 0; i < SMC_RET_NUM; i++) {
> > +                       *(u32 *)buf = val[i];
> > +                       buf += sizeof(u32);
> > +               }
> > +
> > +               retval += MTK_SEC_RND_SIZE;
> > +               max -= MTK_SEC_RND_SIZE;
> > +       }
> > +
> > +       return retval;
> > +}
> > +
> > +static struct hwrng mtk_sec_rng = {
> > +       .name = "mtk_sec_rng",
> > +       .read = mtk_sec_rng_read,
> > +       .quality = 900,
> > +};
> > +
> > +static int mtk_sec_rng_probe(void)
> > +{
> > +       int ret;
> > +
> > +       ret = hwrng_register(&mtk_sec_rng);
> > +       if (ret) {
> > +               pr_err("Failed to register rng device: %d\n", ret);
> > +               return ret;
> > +       }
> > +
> > +       return 0;
> > +}
> > +
> > +static int __init mtk_sec_rng_driver_init(void)
> > +{
> > +       struct device_node *fw_np;
> > +       struct device_node *np;
> > +       const char *method;
> > +
> > +       fw_np = of_find_node_by_name(NULL, "firmware");
> > +       if (!fw_np)
> > +               return -ENODEV;
> > +
> > +       np = of_find_compatible_node(fw_np, NULL, "mediatek,mtk-sec-rng");
> > +       if (!np)
> > +               return -ENODEV;
> > +
> > +       if (of_property_read_string(np, "method", &method))
> > +               return -ENXIO;
> > +
> > +       if (strncmp("smc", method, strlen("smc")))
> > +               return -EINVAL;
> > +
> > +       return mtk_sec_rng_probe();
> > +}
> > +
> > +static void __exit mtk_sec_rng_driver_exit(void)
> > +{
> > +       hwrng_unregister(&mtk_sec_rng);
> > +}
> > +
> > +module_init(mtk_sec_rng_driver_init);
> > +module_exit(mtk_sec_rng_driver_exit);
> > +
> > +MODULE_DESCRIPTION("MediaTek Security Random Number Generator Driver");
> > +MODULE_AUTHOR("Neal Liu <[email-masked]>");
> > +MODULE_LICENSE("GPL");
> > --
> > 1.7.9.5
> 
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