[PATCH v5 3/8] stm32mp1: rng: Add a driver for random number generator(rng) device

[Sughosh Ganu]

On Fri, 27 Dec 2019 at 13:21, Heinrich Schuchardt <xypron.glpk at gmx.de>
wrote:

> On 12/26/19 6:25 PM, Sughosh Ganu wrote:
> > Add a driver for the rng device found on stm32mp1 platforms. The
> > driver provides a routine for reading the random number seed from the
> > hardware device.
> >
> > Signed-off-by: Sughosh Ganu <sughosh.ganu at linaro.org>
> > Reviewed-by: Patrice Chotard <patrice.chotard at st.com>
> > Acked-by: Patrick Delaunay <patrick.delaunay at st.com>
> > ---
> > Changes since V4:
> > * Return number of bytes read on a successful read, and a -ve value on
> >    error.
> >
> >   drivers/rng/Kconfig        |   7 ++
> >   drivers/rng/Makefile       |   1 +
> >   drivers/rng/stm32mp1_rng.c | 165
> +++++++++++++++++++++++++++++++++++++++++++++
> >   3 files changed, 173 insertions(+)
> >   create mode 100644 drivers/rng/stm32mp1_rng.c
> >
> > diff --git a/drivers/rng/Kconfig b/drivers/rng/Kconfig
> > index dd44cc0..c9c4751 100644
> > --- a/drivers/rng/Kconfig
> > +++ b/drivers/rng/Kconfig
> > @@ -5,3 +5,10 @@ config DM_RNG
> >         Enable driver model for random number generator(rng) devices.
> >         This interface is used to initialise the rng device and to
> >         read the random seed from the device.
> > +
> > +config RNG_STM32MP1
> > +     bool "Enable random number generator for STM32MP1"
> > +     depends on ARCH_STM32MP && DM_RNG
> > +     default n
> > +     help
> > +       Enable STM32MP1 rng driver.
> > diff --git a/drivers/rng/Makefile b/drivers/rng/Makefile
> > index 311705b..699beb3 100644
> > --- a/drivers/rng/Makefile
> > +++ b/drivers/rng/Makefile
> > @@ -4,3 +4,4 @@
> >   #
> >
> >   obj-$(CONFIG_DM_RNG) += rng-uclass.o
> > +obj-$(CONFIG_RNG_STM32MP1) += stm32mp1_rng.o
> > diff --git a/drivers/rng/stm32mp1_rng.c b/drivers/rng/stm32mp1_rng.c
> > new file mode 100644
> > index 0000000..56ad848
> > --- /dev/null
> > +++ b/drivers/rng/stm32mp1_rng.c
> > @@ -0,0 +1,165 @@
> > +// SPDX-License-Identifier: GPL-2.0-or-later
> > +/*
> > + * Copyright (c) 2019, Linaro Limited
> > + */
> > +
> > +#include <common.h>
> > +#include <clk.h>
> > +#include <dm.h>
> > +#include <reset.h>
> > +#include <rng.h>
> > +
> > +#include <asm/io.h>
> > +#include <linux/iopoll.h>
> > +#include <linux/kernel.h>
> > +
> > +#define RNG_CR 0x00
> > +#define RNG_CR_RNGEN BIT(2)
> > +#define RNG_CR_CED BIT(5)
> > +
> > +#define RNG_SR 0x04
> > +#define RNG_SR_SEIS BIT(6)
> > +#define RNG_SR_CEIS BIT(5)
> > +#define RNG_SR_SECS BIT(2)
> > +#define RNG_SR_DRDY BIT(0)
> > +
> > +#define RNG_DR 0x08
> > +
> > +struct stm32_rng_platdata {
> > +     fdt_addr_t base;
> > +     struct clk clk;
> > +     struct reset_ctl rst;
> > +};
> > +
> > +static int stm32_rng_read(struct udevice *dev, void *data, size_t len)
> > +{
> > +     int retval = 0, i, nbytes;
> > +     u32 sr, count, reg;
> > +     size_t increment;
> > +     struct stm32_rng_platdata *pdata = dev_get_platdata(dev);
> > +
> > +     /*
> > +      * Only INT_MAX number of bytes can be returned. If more
> > +      * bytes need to be read, the caller must do it in a loop.
> > +      */
> > +     if (len > INT_MAX)
> > +             len = INT_MAX;
> > +
> > +     nbytes = len;
> > +     while (len > 0) {
> > +             retval = readl_poll_timeout(pdata->base + RNG_SR, sr,
> > +                                         sr & RNG_SR_DRDY, 10000);
> > +             if (retval)
> > +                     return retval;
> > +
> > +             if (sr & (RNG_SR_SEIS | RNG_SR_SECS)) {
> > +                     /* As per SoC TRM */
> > +                     clrbits_le32(pdata->base + RNG_SR, RNG_SR_SEIS);
> > +                     for (i = 0; i < 12; i++)
> > +                             readl(pdata->base + RNG_DR);
> > +                     if (readl(pdata->base + RNG_SR) & RNG_SR_SEIS) {
> > +                             printf("RNG Noise");
> > +                             return -EIO;
>
> The SEIS bit indicates a seed error. According to the RM0090 Reference
> manual for the STM32F429/439 you should clear the SEIS bit and set the
> RNGEN bit to restart the RNG.
>
>
> https://www.st.com/content/ccc/resource/technical/document/reference_manual/3d/6d/5a/66/b4/99/40/d4/DM00031020.pdf/files/DM00031020.pdf/jcr:content/translations/en.DM00031020.pdf
> ,
> page 768.
>
> So why do you return an error code here? What do you expect the caller
> to do?
>

I am referring to the stm32mp157 trm, Noise source error detection, pg 1939.


>
> Should we check the CEIS flag which indicates a clock error?
>

The stm32mp1 clk driver in tf-a sets the rng peripheral source to lsi_ck,
which is 32KHz. This is configured after tests done by ST engineers to
provide better entropy. However, with this clock source, the CEIS bit is
set, since Frng is less than (Fhclk/32). This is not an issue though, since
the trm clearly states that the clock error has no impact on the generated
random numbers, and that the application can still read the RNG_DR register.


>
> > +                     }
> > +                     /* start again */
> > +                     continue;
> > +             }
> > +
>
> It took me some time to understand what this loop does. Please, add a
> comment indicating that we copy up to 16 random bytes from the RNG.
>

Ok. Will do.

-sughosh