linux-kernel - Re: [PATCH] kbuild: Disable -Wpointer-to-enum-cast

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Date:   Tue, 10 Mar 2020 09:16:22 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     David Laight <David.Laight@...lab.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        stable <stable@...r.kernel.org>,
        Saravana Kannan <saravanak@...gle.com>
Subject: Re: [PATCH] kbuild: Disable -Wpointer-to-enum-cast

+ Saravana, who I spoke to briefly about this.


On Tue, Mar 10, 2020 at 8:32 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Tue, Mar 10, 2020 at 8:31 PM David Laight <David.Laight@...lab.com> wrote:
> >
> > From: Nathan Chancellor
> > > Sent: 10 March 2020 01:26
> > ...
> > > Sure, I can send v2 to do that but I think that sending 97 patches just
> > > casting the small values (usually less than twenty) to unsigned long
> > > then to the enum is rather frivolous. I audited at least ten to fifteen
> > > of these call sites when creating the clang patch and they are all
> > > basically false positives.
> >
> > Such casts just make the code hard to read.
> > If misused casts can hide horrid bugs.
> > IMHO sprinkling the code with casts just to remove
> > compiler warnings will bite back one day.
> >
>
> I agree that too much casts make the code hard to read,
> but irrespective of this patch, there is no difference
> in the fact that we need a cast to convert
> (const void *) to a non-pointer value.
>
> The difference is whether we use
> (uintptr_t) or (enum foo).
>
>
>
>
> If we want to avoid casts completely,
> we could use union in struct of_device_id
> although this might be rejected.
>
>
> FWIW:
>
> diff --git a/drivers/ata/ahci_brcm.c b/drivers/ata/ahci_brcm.c
> index 6853dbb4131d..534170bea134 100644
> --- a/drivers/ata/ahci_brcm.c
> +++ b/drivers/ata/ahci_brcm.c
> @@ -415,11 +415,11 @@ static struct scsi_host_template ahci_platform_sht = {
>  };
>
>  static const struct of_device_id ahci_of_match[] = {
> -       {.compatible = "brcm,bcm7425-ahci", .data = (void *)BRCM_SATA_BCM7425},
> -       {.compatible = "brcm,bcm7445-ahci", .data = (void *)BRCM_SATA_BCM7445},
> -       {.compatible = "brcm,bcm63138-ahci", .data = (void *)BRCM_SATA_BCM7445},
> -       {.compatible = "brcm,bcm-nsp-ahci", .data = (void *)BRCM_SATA_NSP},
> -       {.compatible = "brcm,bcm7216-ahci", .data = (void *)BRCM_SATA_BCM7216},
> +       {.compatible = "brcm,bcm7425-ahci", .data2 = BRCM_SATA_BCM7425},
> +       {.compatible = "brcm,bcm7445-ahci", .data2 = BRCM_SATA_BCM7445},
> +       {.compatible = "brcm,bcm63138-ahci", .data2 = BRCM_SATA_BCM7445},
> +       {.compatible = "brcm,bcm-nsp-ahci", .data2 = BRCM_SATA_NSP},
> +       {.compatible = "brcm,bcm7216-ahci", .data2 = BRCM_SATA_BCM7216},
>         {},
>  };
>  MODULE_DEVICE_TABLE(of, ahci_of_match);
> @@ -442,7 +442,7 @@ static int brcm_ahci_probe(struct platform_device *pdev)
>         if (!of_id)
>                 return -ENODEV;
>
> -       priv->version = (enum brcm_ahci_version)of_id->data;
> +       priv->version = of_id->data2;
>         priv->dev = dev;
>
>         res = platform_get_resource_byname(pdev, IORESOURCE_MEM, "top-ctrl");
> diff --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
> index e3596db077dc..98d44ebf146a 100644
> --- a/include/linux/mod_devicetable.h
> +++ b/include/linux/mod_devicetable.h
> @@ -261,7 +261,10 @@ struct of_device_id {
>         char    name[32];
>         char    type[32];
>         char    compatible[128];
> -       const void *data;
> +       union {
> +               const void *data;
> +               unsigned long data2;
> +       };
>  };
>

-- 
Thanks,
~Nick Desaulniers