Re: [PATCH 2/3] hwrng: stm32 - add support for STM32 HW RNG

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

On 4 October 2015 at 09:52, Maxime Coquelin <mcoquelin.stm32@gmail.com> wrote:
> Hi Daniel,
>
> On 10/03/2015 10:35 PM, Daniel Thompson wrote:
>>
>> Add support for STMicroelectronics STM32 random number generator.
>>
>> The config value defaults to N, reflecting the fact that STM32 is a
>> very low resource microcontroller platform and unlikely to be targeted
>> by any "grown up" defconfigs.
>>
>> Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
>> ---
>>   drivers/char/hw_random/Kconfig     |  12 +++
>>   drivers/char/hw_random/Makefile    |   1 +
>>   drivers/char/hw_random/stm32-rng.c | 192
>> +++++++++++++++++++++++++++++++++++++
>>   3 files changed, 205 insertions(+)
>>   create mode 100644 drivers/char/hw_random/stm32-rng.c
>
> <snip>
>>
>> diff --git a/drivers/char/hw_random/stm32-rng.c
>> b/drivers/char/hw_random/stm32-rng.c
>> new file mode 100644
>> index 000000000000..37dfa5fca105
>> --- /dev/null
>> +++ b/drivers/char/hw_random/stm32-rng.c
>> @@ -0,0 +1,192 @@
>
> <snip>
>
>> +#include <linux/clk.h>
>> +#include <linux/delay.h>
>> +#include <linux/hw_random.h>
>> +#include <linux/io.h>
>> +#include <linux/kernel.h>
>> +#include <linux/module.h>
>> +#include <linux/of_address.h>
>> +#include <linux/of_platform.h>
>> +#include <linux/slab.h>
>> +
>> +#define RNG_CR 0x00
>> +#define RNG_CR_RNGEN BIT(2)
>> +
>> +#define RNG_SR 0x04
>> +#define RNG_SR_SEIS BIT(6)
>> +#define RNG_SR_CEIS BIT(5)
>> +#define RNG_SR_DRDY BIT(0)
>> +
>> +#define RNG_DR 0x08
>> +
>> +/*
>> + * It takes 40 cycles @ 48MHz to generate each random number (e.g. <1us).
>> + * At the time of writing STM32 parts max out at ~200MHz meaning a
>> timeout
>> + * of 500 leaves us a very comfortable margin for error. The loop to
>> which
>> + * the timeout applies takes at least 4 instructions per cycle so the
>> + * timeout is enough to take us up to multi-GHz parts!
>> + */
>> +#define RNG_TIMEOUT 500
>> +
>> +struct stm32_rng_private {
>> +       struct hwrng rng;
>> +       void __iomem *base;
>> +       struct clk *clk;
>> +};
>> +
>> +static int stm32_rng_read(struct hwrng *rng, void *data, size_t max, bool
>> wait)
>> +{
>> +       struct stm32_rng_private *priv =
>> +           container_of(rng, struct stm32_rng_private, rng);
>> +       u32 cr, sr;
>> +       int retval = 0;
>> +
>> +       /* enable random number generation */
>> +       clk_enable(priv->clk);
>> +       cr = readl(priv->base + RNG_CR);
>> +       writel(cr | RNG_CR_RNGEN, priv->base + RNG_CR);
>> +
>> +       while (max > sizeof(u32)) {
>> +               sr = readl(priv->base + RNG_SR);
>> +               if (!sr && wait) {
>> +                       unsigned int timeout = RNG_TIMEOUT;
>> +
>> +                       do {
>> +                               cpu_relax();
>> +                               sr = readl(priv->base + RNG_SR);
>> +                       } while (!sr && --timeout);
>> +               }
>> +
>> +               /* Has h/ware error dection been triggered? */
>> +               if (WARN_ON(sr & (RNG_SR_SEIS | RNG_SR_CEIS)))
>
> Maybe you should instead use WARN_ONCE?
> Because from what I understand in the datasheet, CEIS bit indicates and
> error with clock configuration.
> If that happens, it is likely the same error will occur each time this
> function will be called.

Ok.


>> +                       break;
>> +
>> +               /* No data ready... */
>> +               if (!sr)
>> +                       break;
>
> Maybe you could perform this check before the error detection, as if !sr,
> the HW error condition will be always false.

I plan to combine both checks since timing out is still a serious
error even if the h/ware didn't detect it.

>>
>> +
>> +               *(u32 *)data = readl(priv->base + RNG_DR);
>> +
>> +               retval += sizeof(u32);
>> +               data += sizeof(u32);
>> +               max -= sizeof(u32);
>> +       }
>> +
>> +       /* disable the generator */
>> +       writel(cr, priv->base + RNG_CR);
>> +       clk_disable(priv->clk);
>> +
>> +       return retval || !wait ? retval : -EIO;
>> +}
>
>
> Couldn't we use "_relaxed" versions of readl/writel?
> I might save some not needed barriers.

Will fix. I had a "historic" reason to avoid _relaxed but this is no
longer relevant.


>> +static int stm32_rng_probe(struct platform_device *ofdev)
>> +{
>> +       struct device *dev = &ofdev->dev;
>> +       struct device_node *np = ofdev->dev.of_node;
>> +       struct stm32_rng_private *priv;
>> +       struct resource res;
>> +       int err;
>> +
>> +       priv = devm_kzalloc(dev, sizeof(struct stm32_rng_private),
>> GFP_KERNEL);
>> +       if (!priv)
>> +               return -ENOMEM;
>> +
>> +       err = of_address_to_resource(np, 0, &res);
>> +       if (err)
>> +               return err;
>> +
>> +       priv->base = devm_ioremap_resource(dev, &res);
>> +       if (IS_ERR(priv->base))
>> +               return PTR_ERR(priv->base);
>> +
>> +       priv->clk = devm_clk_get(&ofdev->dev, NULL);
>> +       if (IS_ERR(priv->clk))
>> +               return PTR_ERR(priv->clk);
>> +
>> +       dev_set_drvdata(dev, priv);
>> +
>> +       priv->rng.name = dev_driver_string(dev),
>> +       priv->rng.init = stm32_rng_init,
>> +       priv->rng.cleanup = stm32_rng_cleanup,
>> +       priv->rng.read = stm32_rng_read,
>> +       priv->rng.priv = (unsigned long) dev;
>> +
>> +       err = hwrng_register(&priv->rng);
>> +       if (err)
>> +               return err;
>> +
>> +       return 0;
>
> As detected with Coccinelle by Fengguang build system, you could simplify:
> return hwrng_register(&priv->rng);

Will do.


Thanks

Daniel.