| Message ID | 20190307080038.3727426-1-arnd@arndb.de |
|---|---|
| State | Superseded |
| Headers | show |
| Series | dma-mapping: work around clang bug | expand |
Hi Arnd, On Thu, Mar 7, 2019 at 9:01 AM Arnd Bergmann <arnd@arndb.de> wrote: > Clang has a rather annoying behavior of checking for integer > arithmetic problems in code paths that are discarded by gcc > before that perfoms the same checks. > > For DMA_BIT_MASK(64), this leads to a warning despite the > result of the macro being completely sensible: > > arch/arm/plat-iop/adma.c:146:24: error: shift count >= width of type [-Werror,-Wshift-count-overflow] > .coherent_dma_mask = DMA_BIT_MASK(64), > > The best workaround I could come up with is to shift the > value twice, which makes the macro way less readable but > always has the same result. > > Link: https://bugs.llvm.org/show_bug.cgi?id=38789 > Signed-off-by: Arnd Bergmann <arnd@arndb.de> > --- > include/linux/dma-mapping.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h > index 75e60be91e5f..380d3a95d02e 100644 > --- a/include/linux/dma-mapping.h > +++ b/include/linux/dma-mapping.h > @@ -138,7 +138,8 @@ struct dma_map_ops { > extern const struct dma_map_ops dma_virt_ops; > extern const struct dma_map_ops dma_dummy_ops; > > -#define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1)) > +/* double shift to work around https://bugs.llvm.org/show_bug.cgi?id=38789 */ > +#define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : (((1ULL<<((n)-1))-1) << 1)) The second "-1" should be done on the final result, not on the intermediate value. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
On Thu, Mar 7, 2019 at 9:28 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote: > On Thu, Mar 7, 2019 at 9:01 AM Arnd Bergmann <arnd@arndb.de> wrote: > > Clang has a rather annoying behavior of checking for integer > > arithmetic problems in code paths that are discarded by gcc > > before that perfoms the same checks. > > > > For DMA_BIT_MASK(64), this leads to a warning despite the > > result of the macro being completely sensible: > > > > arch/arm/plat-iop/adma.c:146:24: error: shift count >= width of type [-Werror,-Wshift-count-overflow] > > .coherent_dma_mask = DMA_BIT_MASK(64), > > > > The best workaround I could come up with is to shift the > > value twice, which makes the macro way less readable but > > always has the same result. > > > > Link: https://bugs.llvm.org/show_bug.cgi?id=38789 > > Signed-off-by: Arnd Bergmann <arnd@arndb.de> > > --- > > include/linux/dma-mapping.h | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h > > index 75e60be91e5f..380d3a95d02e 100644 > > --- a/include/linux/dma-mapping.h > > +++ b/include/linux/dma-mapping.h > > @@ -138,7 +138,8 @@ struct dma_map_ops { > > extern const struct dma_map_ops dma_virt_ops; > > extern const struct dma_map_ops dma_dummy_ops; > > > > -#define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1)) > > +/* double shift to work around https://bugs.llvm.org/show_bug.cgi?id=38789 */ > > +#define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : (((1ULL<<((n)-1))-1) << 1)) > > The second "-1" should be done on the final result, not on the > intermediate value. Ah, of course. I'll send an update patch in a bit, sorry about this. Arnd
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h index 75e60be91e5f..380d3a95d02e 100644 --- a/include/linux/dma-mapping.h +++ b/include/linux/dma-mapping.h @@ -138,7 +138,8 @@ struct dma_map_ops { extern const struct dma_map_ops dma_virt_ops; extern const struct dma_map_ops dma_dummy_ops; -#define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1)) +/* double shift to work around https://bugs.llvm.org/show_bug.cgi?id=38789 */ +#define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : (((1ULL<<((n)-1))-1) << 1)) #define DMA_MASK_NONE 0x0ULL
Clang has a rather annoying behavior of checking for integer arithmetic problems in code paths that are discarded by gcc before that perfoms the same checks. For DMA_BIT_MASK(64), this leads to a warning despite the result of the macro being completely sensible: arch/arm/plat-iop/adma.c:146:24: error: shift count >= width of type [-Werror,-Wshift-count-overflow] .coherent_dma_mask = DMA_BIT_MASK(64), The best workaround I could come up with is to shift the value twice, which makes the macro way less readable but always has the same result. Link: https://bugs.llvm.org/show_bug.cgi?id=38789 Signed-off-by: Arnd Bergmann <arnd@arndb.de> --- include/linux/dma-mapping.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) -- 2.20.0