netdev - Re: [PATCH v2] net: stmmac: Avoid sometimes uninitialized Clang warnings

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Date:   Thu, 7 Mar 2019 10:12:58 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v2] net: stmmac: Avoid sometimes uninitialized Clang warnings

On Thu, Mar 7, 2019 at 10:00 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> When building with -Wsometimes-uninitialized, Clang warns:
>
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:495:3: warning: variable 'ns' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:495:3: warning: variable 'ns' is used uninitialized whenever '&&' condition is false [-Wsometimes-uninitialized]
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:532:3: warning: variable 'ns' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:532:3: warning: variable 'ns' is used uninitialized whenever '&&' condition is false [-Wsometimes-uninitialized]
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:741:3: warning: variable 'sec_inc' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:741:3: warning: variable 'sec_inc' is used uninitialized whenever '&&' condition is false [-Wsometimes-uninitialized]
>
> Clang is concerned with the use of stmmac_do_void_callback (which
> stmmac_get_timestamp and stmmac_config_sub_second_increment wrap),
> as it may fail to initialize these values if the if condition was ever
> false (meaning the callbacks don't exist). It's not wrong because the
> callbacks (get_timestamp and config_sub_second_increment respectively)
> are the ones that initialize the variables. While it's unlikely that the
> callbacks are ever going to disappear and make that condition false, we
> can easily avoid this warning by zero initialize the variables.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/384
> Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

V2 LGTM, thanks for resending.

> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>
> v1 -> v2:
>
> * Respect reverse Christmas tree variable ordering in
>   stmmac_hwtstamp_set.
>
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index e2a13ec2e30b..97c5e1aad88f 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -480,7 +480,7 @@ static void stmmac_get_tx_hwtstamp(struct stmmac_priv *priv,
>                                    struct dma_desc *p, struct sk_buff *skb)
>  {
>         struct skb_shared_hwtstamps shhwtstamp;
> -       u64 ns;
> +       u64 ns = 0;
>
>         if (!priv->hwts_tx_en)
>                 return;
> @@ -519,7 +519,7 @@ static void stmmac_get_rx_hwtstamp(struct stmmac_priv *priv, struct dma_desc *p,
>  {
>         struct skb_shared_hwtstamps *shhwtstamp = NULL;
>         struct dma_desc *desc = p;
> -       u64 ns;
> +       u64 ns = 0;
>
>         if (!priv->hwts_rx_en)
>                 return;
> @@ -564,8 +564,8 @@ static int stmmac_hwtstamp_set(struct net_device *dev, struct ifreq *ifr)
>         u32 snap_type_sel = 0;
>         u32 ts_master_en = 0;
>         u32 ts_event_en = 0;
> +       u32 sec_inc = 0;
>         u32 value = 0;
> -       u32 sec_inc;
>         bool xmac;
>
>         xmac = priv->plat->has_gmac4 || priv->plat->has_xgmac;
> --
> 2.21.0
>
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-- 
Thanks,
~Nick Desaulniers