Re: [PATCH 2/8] drm/i915: Added SDP and VSC structures for handling PSR for eDP

[Philipp Klaus Krause <pkk@spth.de>]

Am 14.01.2013 12:22, schrieb Ville Syrjälä:
> On Fri, Jan 11, 2013 at 09:15:40PM +0100, Daniel Vetter wrote:
>> On Fri, Jan 11, 2013 at 05:57:51PM -0200, Rodrigo Vivi wrote:
>>> From: Shobhit Kumar <shobhit.kumar@intel.com>
>>>
>>> Signed-off-by: Sateesh Kavuri <sateesh.kavuri@intel.com>
>>>
>>> v2: Modified and corrected the structures to be more in line for
>>> kernel coding guidelines and rebased the code on Paulo's DP patchset
>>>
>>> Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
>>>
>>> v3: removing unecessary identation at DP_RECEIVER_CAP_SIZE
>>>
>>> Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
>>
>> DP spec stuff should be put into the drm core imo, i.e. into
>> include/drm/drm_dp_helper.h Since all bitfields are u8 we also have no
>> problems with endianess for this.
> 
> I don't think C guarantees anything about the order of bits inside
> bitfieds.
> 

C11 standard, section 6.7.2.1:

"An implementation may allocate any addressable storage unit large
enough to hold a bit-field. If enough space remains, a bit-field that
immediately follows another bit-field in a structure shall be packed
into adjacent bits of the same unit. If insufficient space remains,
whether a bit-field that does not fit is put into the next unit or
overlaps adjacent units is implementation-defined. The order of
allocation of bit-fields within a unit (high-order to low-order or
low-order to high-order) is implementation-defined. The alignment of the
addressable storage unit is unspecified."

C doesn't guarantee that your u8 is a valid type for bit-fields. The
only types a C compiler must support for bit-fields are bool, int,
signed int and unsigned int (note that bit-fields are also the only
place where the type int can be different from signed int).

Philipp