RE: [PATCH v2] staging/comedi/dt282x: avoid integer overflow warning
From: Hartley Sweeten
Date: Thu Mar 17 2016 - 11:48:25 EST
On Wednesday, March 16, 2016 1:51 PM, Arnd Bergmann wrote:
> gcc-6 warns about passing negative signed integer into swab16()
> in the dt282x driver:
> drivers/staging/comedi/drivers/dt282x.c: In function 'dt282x_load_changain':
> include/uapi/linux/swab.h:14:33: warning: integer overflow in expression [-Woverflow]
> (((__u16)(x) & (__u16)0xff00U) >> 8)))
> include/uapi/linux/swab.h:107:2: note: in expansion of macro '___constant_swab16'
> ___constant_swab16(x) : \
> include/uapi/linux/byteorder/big_endian.h:34:43: note: in expansion of macro '__swab16'
> #define __cpu_to_le16(x) ((__force __le16)__swab16((x)))
> include/linux/byteorder/generic.h:89:21: note: in expansion of macro '__cpu_to_le16'
> #define cpu_to_le16 __cpu_to_le16
> arch/arm/include/asm/io.h:250:6: note: in expansion of macro 'cpu_to_le16'
> cpu_to_le16(v),__io(p)); })
> drivers/staging/comedi/drivers/dt282x.c:566:2: note: in expansion of macro 'outw'
> outw(DT2821_CHANCSR_LLE | DT2821_CHANCSR_NUMB(n),
Is this a gcc-6 specific issue? Seems line this warning should be showing
up in a lot of drivers.
> The warning makes sense, though the code is correct as far as I
> can tell.
> This disambiguates the operation by making the constant expressions
> we pass here explicitly 'unsigned', which helps to avoid the warning.
> As pointed out by Hartley Sweeten, scripts/checkpatch.pl notices that
> the shifts here are rather unreadable, though the suggested BIT()
> macro wouldn't work either. I'm changing it to a hexadecimal notation,
> which hopefully improves readability. I'm leaving the DT2821_CHANCSR_PRESLA
> alone because it seems wrong.
BIT() should work for the ones pointed out by checpatch.pl.
I would argue that the hexadecimal notation is still rather unreadable.
These ones make my head hurt...
-#define DT2821_ADCSR_GS(x) (((x) & 0x3) << 4)
+#define DT2821_ADCSR_GS(x) (0x0030u & ((x) << 4))
-#define DT2821_DACSR_YSEL(x) ((x) << 9)
+#define DT2821_DACSR_YSEL(x) (0x7e00u & (x) << 9)
-#define DT2821_SUPCSR_DS_PIO (0 << 10)
-#define DT2821_SUPCSR_DS_AD_CLK (1 << 10)
-#define DT2821_SUPCSR_DS_DA_CLK (2 << 10)
-#define DT2821_SUPCSR_DS_AD_TRIG (3 << 10)
+#define DT2821_SUPCSR_DS_PIO (0x0c00u & (0u << 10))
+#define DT2821_SUPCSR_DS_AD_CLK (0x0c00u & (1u << 10))
+#define DT2821_SUPCSR_DS_DA_CLK (0x0c00u & (2u << 10))
+#define DT2821_SUPCSR_DS_AD_TRIG (0x0c00u & (3u << 10))
Also, most of the comedi drivers use the BIT() macro. Are you planning on
changing all of them to use hexadecimal notation?