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

On Tue, Sep 28, 2021 at 7:15 AM Arnd Bergmann [off-list ref] wrote:

On Tue, Sep 28, 2021 at 2:00 AM Bailey Forrest [off-list ref] wrote:

quoted

On Mon, Sep 27, 2021 at 4:21 PM Jakub Kicinski [off-list ref] wrote:

quoted

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:

quoted

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.