Re: [PATCH v1 2/2] hwrng: npcm: add NPCM RNG driver

[Daniel Thompson <daniel.thompson@linaro.org>]

On Mon, Sep 09, 2019 at 05:31:30PM +0300, Tomer Maimon wrote:
> Hi Daniel,
> 
> appreciate your comments and sorry for the late reply
> 
> On Thu, 29 Aug 2019 at 13:47, Daniel Thompson <daniel.thompson@linaro.org>
> wrote:
> 
> > On Wed, Aug 28, 2019 at 07:26:17PM +0300, Tomer Maimon wrote:
> > > Add Nuvoton NPCM BMC Random Number Generator(RNG) driver.
> > >
> > > Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
> > > ---
> > >  drivers/char/hw_random/Kconfig    |  13 ++
> > >  drivers/char/hw_random/Makefile   |   1 +
> > >  drivers/char/hw_random/npcm-rng.c | 207 ++++++++++++++++++++++++++++++
> > >  3 files changed, 221 insertions(+)
> > >  create mode 100644 drivers/char/hw_random/npcm-rng.c
> > >
> > > diff --git a/drivers/char/hw_random/npcm-rng.c
> > b/drivers/char/hw_random/npcm-rng.c
> > > new file mode 100644
> > > index 000000000000..5b4b1b6cb362
> > > --- /dev/null
> > > +++ b/drivers/char/hw_random/npcm-rng.c
> > > @@ -0,0 +1,207 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +// Copyright (c) 2019 Nuvoton Technology corporation.
> > > +
> > > +#include <linux/kernel.h>
> > > +#include <linux/module.h>
> > > +#include <linux/io.h>
> > > +#include <linux/iopoll.h>
> > > +#include <linux/init.h>
> > > +#include <linux/random.h>
> > > +#include <linux/err.h>
> > > +#include <linux/platform_device.h>
> > > +#include <linux/hw_random.h>
> > > +#include <linux/delay.h>
> > > +#include <linux/of_irq.h>
> > > +#include <linux/pm_runtime.h>
> > > +
> > > +#define NPCM_RNGCS_REG               0x00    /* Control and status
> > register */
> > > +#define NPCM_RNGD_REG                0x04    /* Data register */
> > > +#define NPCM_RNGMODE_REG     0x08    /* Mode register */
> > > +
> > > +#define NPCM_RNG_CLK_SET_25MHZ       GENMASK(4, 3) /* 20-25 MHz */
> > > +#define NPCM_RNG_DATA_VALID  BIT(1)
> > > +#define NPCM_RNG_ENABLE              BIT(0)
> > > +#define NPCM_RNG_M1ROSEL     BIT(1)
> > > +
> > > +#define NPCM_RNG_TIMEOUT_POLL        20
> >
> > Might be better to define this in real-world units (such as
> > milliseconds) since the timeout is effectively the longest time the
> > hardware can take to generate 4 bytes.
> >
> > > +
> > > +#define to_npcm_rng(p)       container_of(p, struct npcm_rng, rng)
> > > +
> > > +struct npcm_rng {
> > > +     void __iomem *base;
> > > +     struct hwrng rng;
> > > +};
> > > +
> > > +static int npcm_rng_init(struct hwrng *rng)
> > > +{
> > > +     struct npcm_rng *priv = to_npcm_rng(rng);
> > > +     u32 val;
> > > +
> > > +     val = readl(priv->base + NPCM_RNGCS_REG);
> > > +     val |= NPCM_RNG_ENABLE;
> > > +     writel(val, priv->base + NPCM_RNGCS_REG);
> > > +
> > > +     return 0;
> > > +}
> > > +
> > > +static void npcm_rng_cleanup(struct hwrng *rng)
> > > +{
> > > +     struct npcm_rng *priv = to_npcm_rng(rng);
> > > +     u32 val;
> > > +
> > > +     val = readl(priv->base + NPCM_RNGCS_REG);
> > > +     val &= ~NPCM_RNG_ENABLE;
> > > +     writel(val, priv->base + NPCM_RNGCS_REG);
> > > +}
> > > +
> > > +static bool npcm_rng_wait_ready(struct hwrng *rng, bool wait)
> > > +{
> > > +     struct npcm_rng *priv = to_npcm_rng(rng);
> > > +     int timeout_cnt = 0;
> > > +     int ready;
> > > +
> > > +     ready = readl(priv->base + NPCM_RNGCS_REG) & NPCM_RNG_DATA_VALID;
> > > +     while ((ready == 0) && (timeout_cnt < NPCM_RNG_TIMEOUT_POLL)) {
> > > +             usleep_range(500, 1000);
> > > +             ready = readl(priv->base + NPCM_RNGCS_REG) &
> > > +                     NPCM_RNG_DATA_VALID;
> > > +             timeout_cnt++;
> > > +     }
> > > +
> > > +     return !!ready;
> > > +}
> >
> > This looks like an open-coded version of readl_poll_timeout()... better
> > to use the library function.
> >
> > Also the sleep looks a bit long to me. What is the generation rate of
> > the peripheral? Most RNG drivers have short intervals between data
> > generation so they use delays rather than sleeps (a.k.a.
> > readl_poll_timeout_atomic() ).
>
> the HWRNG generate byte of random data in a few milliseconds so it is
> better to use the sleep command.

That's fine, just use readl_poll_timeout() then.


> > > +
> > > +static int npcm_rng_read(struct hwrng *rng, void *buf, size_t max, bool
> > wait)
> > > +{
> > > +     struct npcm_rng *priv = to_npcm_rng(rng);
> > > +     int retval = 0;
> > > +
> > > +     pm_runtime_get_sync((struct device *)priv->rng.priv);
> > > +
> > > +     while (max >= sizeof(u32)) {
> > > +             if (!npcm_rng_wait_ready(rng, wait))
> > > +                     break;
> >
> > The code as currently written does not honour the wait parameter (e.g.
> > it sleeps even when wait is false).
> >
> >
> > > +
> > > +             *(u32 *)buf = readl(priv->base + NPCM_RNGD_REG);
> > > +             retval += sizeof(u32);
> > > +             buf += sizeof(u32);
> > > +             max -= sizeof(u32);
> > > +     }
> > > +
> > > +     pm_runtime_mark_last_busy((struct device *)priv->rng.priv);
> > > +     pm_runtime_put_sync_autosuspend((struct device *)priv->rng.priv);
> > > +
> > > +     return retval || !wait ? retval : -EIO;
> > > +}
> > > +
> > > +static int npcm_rng_probe(struct platform_device *pdev)
> > > +{
> > > +     struct npcm_rng *priv;
> > > +     struct resource *res;
> > > +     u32 quality;
> > > +     int ret;
> > > +
> > > +     priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
> > > +     if (!priv)
> > > +             return -ENOMEM;
> > > +
> > > +     res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> > > +     priv->base = devm_ioremap_resource(&pdev->dev, res);
> > > +     if (IS_ERR(priv->base))
> > > +             return PTR_ERR(priv->base);
> > > +
> > > +     priv->rng.name = pdev->name;
> > > +#ifndef CONFIG_PM
> > > +     priv->rng.init = npcm_rng_init;
> > > +     priv->rng.cleanup = npcm_rng_cleanup;
> > > +#endif
> > > +     priv->rng.read = npcm_rng_read;
> > > +     priv->rng.priv = (unsigned long)&pdev->dev;
> > > +     if (of_property_read_u32(pdev->dev.of_node, "quality", &quality))
> > > +             priv->rng.quality = 1000;
> > > +     else
> > > +             priv->rng.quality = quality;
> > > +
> > > +     writel(NPCM_RNG_M1ROSEL, priv->base + NPCM_RNGMODE_REG);
> > > +#ifndef CONFIG_PM
> > > +     writel(NPCM_RNG_CLK_SET_25MHZ, priv->base + NPCM_RNGCS_REG);
> > > +#else
> > > +     writel(NPCM_RNG_CLK_SET_25MHZ | NPCM_RNG_ENABLE,
> > > +            priv->base + NPCM_RNGCS_REG);
> > > +#endif
> >
> > If this initialization was moved to npcm_rng_init() then there would be
> > no need for the additional ifdefing. It would also get rid of the
> > (potentially slow) readl calls on the PM wakeup path.
> >
> 
> But when the Kernel have PM configuration than the priv->rng.init is not
> set and
> *add_early_randomness* function is called. for the *add_early_randomness*
> success
> the hwrng need to enabled in the probe.

Sorry but I don't understand this reply.

When CONFIG_PM is enabled then the probe function does not currently set
NPCM_RNG_ENABLE; instead is relies on npcm_rng_init() being called by
the PM logic (as part of pm_runtime_get_sync() ).

Given the code *already* relies on npcm_rng_init() being called by the
PM logic why does it matter if additional init is put there?


Daniel.