On 2/21/19 4:20 PM, Iustin Pop wrote: > Negative constant left-shift is undefined behaviour in the C standard, > and as such newer versions of clang (at least) warn against it. GCC > supports it for a long time, but it would be better to remove it and > rely on defined behaviour. > > My understanding is "~(-1 << N)" in 2's complement is intended to > generate a bit pattern of zeroes ending with N '1' bits. The same can > be achieved by "(1 << N) - 1" in a well-defined way, so switch to it > to remove the warning. > > Tested: building a kernel with generic SCSI tape, and checking basic > operations (mt status, mt eject) on a real LTO unit. Cannot test > the osst driver. > > Signed-off-by: Iustin Pop <iustin@xxxxxxxxx> > --- > (with apologies if I made mistakes in the CC list) > drivers/scsi/osst.c | 2 +- > drivers/scsi/st.c | 2 +- > 2 files changed, 2 insertions(+), 2 deletions(-) > > diff --git a/drivers/scsi/osst.c b/drivers/scsi/osst.c > index 664c1238a87f..be3c73ebbfde 100644 > --- a/drivers/scsi/osst.c > +++ b/drivers/scsi/osst.c > @@ -139,7 +139,7 @@ static int debugging = 1; > #define OSST_TIMEOUT (200 * HZ) > #define OSST_LONG_TIMEOUT (1800 * HZ) > > -#define TAPE_NR(x) (iminor(x) & ~(-1 << ST_MODE_SHIFT)) > +#define TAPE_NR(x) (iminor(x) & ((1 << ST_MODE_SHIFT)-1)) > #define TAPE_MODE(x) ((iminor(x) & ST_MODE_MASK) >> ST_MODE_SHIFT) > #define TAPE_REWIND(x) ((iminor(x) & 0x80) == 0) > #define TAPE_IS_RAW(x) (TAPE_MODE(x) & (ST_NBR_MODES >> 1)) > diff --git a/drivers/scsi/st.c b/drivers/scsi/st.c > index 7ff22d3f03e3..eca84caed6ff 100644 > --- a/drivers/scsi/st.c > +++ b/drivers/scsi/st.c > @@ -169,7 +169,7 @@ static int debugging = DEBUG; > > /* Remove mode bits and auto-rewind bit (7) */ > #define TAPE_NR(x) ( ((iminor(x) & ~255) >> (ST_NBR_MODE_BITS + 1)) | \ > - (iminor(x) & ~(-1 << ST_MODE_SHIFT)) ) > + (iminor(x) & ((1 << ST_MODE_SHIFT)-1))) > #define TAPE_MODE(x) ((iminor(x) & ST_MODE_MASK) >> ST_MODE_SHIFT) > > /* Construct the minor number from the device (d), mode (m), and non-rewind (n) data */ > Reviewed-by: Lee Duncan <lduncan@xxxxxxxx>
