On Mon, 30 May 2016, Chris Wilson chris@chris-wilson.co.uk wrote:
On Mon, May 30, 2016 at 11:32:59AM +0100, David Binderman wrote:
Hello there,
drivers/gpu/drm/i915/i915_reg.h:90:28: warning: result of ‘65535 << 20’ requires 37 bits to represent, but ‘int’ only has 32 bits [-Wshift-overflow=]
Source code is
#define BSM_MASK (0xFFFF << 20)
Maybe better code
#define BSM_MASK (((unsigned long) 0xFFFF) << 20)
#define BSM_MASK (~0u << 20)
It should be a 32bit mask. The current (with the exception of undefined behaviour of shifting into the signbit, fortunately gcc does what we expect) code is functionally current as the mask will be truncated to 32bits.
In the patch I used BSM_MASK (0xFFF << 20). It does have the UB of shifting into the sign bit, but then we have loads of e.g. (1 << 31) in i915_reg.h which is no different.
The original code before e10fa551ae37b had ~((1<<20) - 1).
BR, Jani.