netdev - Re: [PATCH net] gve: DQO: Suppress unused var warnings

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Date:   Tue, 28 Sep 2021 13:04:07 -0700
From:   Bailey Forrest <bcf@...gle.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH net] gve: DQO: Suppress unused var warnings

On Tue, Sep 28, 2021 at 7:15 AM Arnd Bergmann <arnd@...nel.org> wrote:
>
> On Tue, Sep 28, 2021 at 2:00 AM Bailey Forrest <bcf@...gle.com> wrote:
> > On Mon, Sep 27, 2021 at 4:21 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > > On Mon, 27 Sep 2021 13:21:30 -0700 Bailey Forrest wrote:
> > >
> > > Looks like fixing this on the wrong end, dma_unmap_len_set()
> > > and friends should always evaluate their arguments.
> >
> > That makes sense.
> >
> > Arnd, if you want to fix this inside of the dma_* macros, the
> > following diff resolves the errors reported for this driver:
>
> > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> > index dca2b1355bb1..f51eee0f678e 100644
> > --- a/include/linux/dma-mapping.h
> > +++ b/include/linux/dma-mapping.h
> > @@ -590,10 +590,21 @@ static inline int dma_mmap_wc(struct device *dev,
> >  #else
> >  #define DEFINE_DMA_UNMAP_ADDR(ADDR_NAME)
> >  #define DEFINE_DMA_UNMAP_LEN(LEN_NAME)
> > -#define dma_unmap_addr(PTR, ADDR_NAME)           (0)
> > -#define dma_unmap_addr_set(PTR, ADDR_NAME, VAL)  do { } while (0)
> > -#define dma_unmap_len(PTR, LEN_NAME)             (0)
> > -#define dma_unmap_len_set(PTR, LEN_NAME, VAL)    do { } while (0)
> > +
> > +#define dma_unmap_addr(PTR, ADDR_NAME) ({ (void)PTR; 0; })
> > +
> > +#define dma_unmap_addr_set(PTR, ADDR_NAME, VAL) do { \
>
> Unfortunately, this breaks every other driver using these macros, as the
> point of them is that the unmap-address is not actually defined
> and not taking up space in data structure. Referencing it by name
> is a compile-time bug.

My patch works with a small modification:

```
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index dca2b1355bb1..04ca5467562e 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -590,10 +590,10 @@ static inline int dma_mmap_wc(struct device *dev,
 #else
 #define DEFINE_DMA_UNMAP_ADDR(ADDR_NAME)
 #define DEFINE_DMA_UNMAP_LEN(LEN_NAME)
-#define dma_unmap_addr(PTR, ADDR_NAME)           (0)
-#define dma_unmap_addr_set(PTR, ADDR_NAME, VAL)  do { } while (0)
-#define dma_unmap_len(PTR, LEN_NAME)             (0)
-#define dma_unmap_len_set(PTR, LEN_NAME, VAL)    do { } while (0)
+#define dma_unmap_addr(PTR, ADDR_NAME) ({ (void)PTR; 0; })
+#define dma_unmap_addr_set(PTR, ADDR_NAME, VAL) do { (void)PTR; } while (0)
+#define dma_unmap_len(PTR, LEN_NAME) ({ (void)PTR; 0; })
+#define dma_unmap_len_set(PTR, LEN_NAME, VAL) do { (void)PTR; } while (0)
 #endif

 #endif /* _LINUX_DMA_MAPPING_H */
```


To me, this is still the preferred solution. However, your latest
patch looked fine to me.