linux-kernel - Re: [PATCH] hwrng: stm32

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Message-ID: <CALszF6AxXY0LK5fToFg3095+gz6xcZz8KHaYqc=o39vjpBYDmA@mail.gmail.com>
Date:	Tue, 24 May 2016 09:59:41 +0200
From:	Maxime Coquelin <mcoquelin.stm32@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Sudip Mukherjee <sudipm.mukherjee@...il.com>,
	Matt Mackall <mpm@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org
Subject: Re: [PATCH] hwrng: stm32 - fix build warning

2016-05-23 22:35 GMT+02:00 Arnd Bergmann <arnd@...db.de>:
> On Monday, May 23, 2016 6:14:08 PM CEST Sudip Mukherjee wrote:
>> We have been getting build warning about:
>> drivers/char/hw_random/stm32-rng.c: In function 'stm32_rng_read':
>> drivers/char/hw_random/stm32-rng.c:82:19: warning: 'sr' may be used
>>                                       uninitialized in this function
>>
>> On checking the code it turns out that sr can never be used
>> uninitialized as sr is getting initialized in the while loop and while
>> loop will always execute as the minimum value of max can be 32.
>> So just initialize sr to 0 while declaring it to silence the compiler.
>>
>> Signed-off-by: Sudip Mukherjee <sudip.mukherjee@...ethink.co.uk>
>> ---
>
> I notice that you are using a really old compiler. While this warning
> seems to be valid in the sense that the compiler should figure out that
> the variable might be used uninitialized, please update your toolchain
> before reporting other such problems, as gcc-4.6 had a lot more false
> positives that newer ones (5.x or 6.x) have.
>
>>
>> build log at:
>> https://travis-ci.org/sudipm-mukherjee/parport/jobs/132180906
>>
>>  drivers/char/hw_random/stm32-rng.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/char/hw_random/stm32-rng.c b/drivers/char/hw_random/stm32-rng.c
>> index 92a8106..0533370 100644
>> --- a/drivers/char/hw_random/stm32-rng.c
>> +++ b/drivers/char/hw_random/stm32-rng.c
>> @@ -52,7 +52,7 @@ 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 sr;
>> +     u32 sr = 0;
>>       int retval = 0;
>>
>>       pm_runtime_get_sync((struct device *) priv->rng.priv);
>
> Does this work as well?
>
> diff --git a/drivers/char/hw_random/stm32-rng.c b/drivers/char/hw_random/stm32-rng.c
> index 92a810648bd0..5c836b0afa40 100644
> --- a/drivers/char/hw_random/stm32-rng.c
> +++ b/drivers/char/hw_random/stm32-rng.c
> @@ -79,7 +79,7 @@ static int stm32_rng_read(struct hwrng *rng, void *data, size_t max, bool wait)
>                 max -= sizeof(u32);
>         }
>
> -       if (WARN_ONCE(sr & (RNG_SR_SEIS | RNG_SR_CEIS),
> +       if (WARN_ONCE(retval > 0 && (sr & (RNG_SR_SEIS | RNG_SR_CEIS)),
>                       "bad RNG status - %x\n", sr))
>                 writel_relaxed(0, priv->base + RNG_SR);
>
> I think it would be nicer to not add a bogus initialization.
Hmm, no sure this nicer.
The while loop can break before retval is incremented when sr value is
not expected (sr != RNG_SR_DRDY).
In that case, we certainly want to print sr value.

Maybe the better way is just to initialize sr with status register content?

diff --git a/drivers/char/hw_random/stm32-rng.c
b/drivers/char/hw_random/stm32-rng.c
index 92a810648bd0..07a6659d0fe6 100644
--- a/drivers/char/hw_random/stm32-rng.c
+++ b/drivers/char/hw_random/stm32-rng.c
@@ -57,8 +57,8 @@ static int stm32_rng_read(struct hwrng *rng, void
*data, size_t max, bool wait)

        pm_runtime_get_sync((struct device *) priv->rng.priv);

+       sr = readl_relaxed(priv->base + RNG_SR);
        while (max > sizeof(u32)) {
-               sr = readl_relaxed(priv->base + RNG_SR);
                if (!sr && wait) {
                        unsigned int timeout = RNG_TIMEOUT;

@@ -77,6 +77,8 @@ static int stm32_rng_read(struct hwrng *rng, void
*data, size_t max, bool wait)
                retval += sizeof(u32);
                data += sizeof(u32);
                max -= sizeof(u32);
+
+               sr = readl_relaxed(priv->base + RNG_SR);
        }

        if (WARN_ONCE(sr & (RNG_SR_SEIS | RNG_SR_CEIS),


Regards,
Maxime