Hi David, On 13/03/18 13:38, David Laight wrote: > From: Kieran Bingham [mailto:kieran.bingham+renesas@xxxxxxxxxxxxxxxx] >> On 13/03/18 11:20, David Laight wrote: >>> From: Kieran Bingham >>>> Sent: 09 March 2018 22:04 >>>> The kernel provides a __packed definition to abstract away from the >>>> compiler specific attributes tag. >>>> >>>> Convert all packed structures in VSP1 to use it. >>>> >>>> Signed-off-by: Kieran Bingham <kieran.bingham+renesas@xxxxxxxxxxxxxxxx> >>>> --- >>>> drivers/media/platform/vsp1/vsp1_dl.c | 6 +++--- >>>> 1 file changed, 3 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/drivers/media/platform/vsp1/vsp1_dl.c b/drivers/media/platform/vsp1/vsp1_dl.c >>>> index 37e2c984fbf3..382e45c2054e 100644 >>>> --- a/drivers/media/platform/vsp1/vsp1_dl.c >>>> +++ b/drivers/media/platform/vsp1/vsp1_dl.c >>>> @@ -29,19 +29,19 @@ >>>> struct vsp1_dl_header_list { >>>> u32 num_bytes; >>>> u32 addr; >>>> -} __attribute__((__packed__)); >>>> +} __packed; >>>> >>>> struct vsp1_dl_header { >>>> u32 num_lists; >>>> struct vsp1_dl_header_list lists[8]; >>>> u32 next_header; >>>> u32 flags; >>>> -} __attribute__((__packed__)); >>>> +} __packed; >>>> >>>> struct vsp1_dl_entry { >>>> u32 addr; >>>> u32 data; >>>> -} __attribute__((__packed__)); >>>> +} __packed; >>> >>> Do these structures ever actually appear in misaligned memory? >>> If they don't then they shouldn't be marked 'packed'. >> >> I believe the declaration is to ensure that the struct definition is not altered >> by the compiler as these structures specifically define the layout of how the >> memory is read by the VSP1 hardware. > > The C language and ABI define structure layouts. > >> Certainly 2 u32's sequentially stored in a struct are unlikely to be moved or >> rearranged by the compiler (though I believe it would be free to do so if it >> chose without this attribute), but I think the declaration shows the intent of >> the memory structure. > > The language requires the fields be in order and the ABI stops the compiler > adding 'random' padding. > >> Isn't this a common approach throughout the kernel when dealing with hardware >> defined memory structures ? > > Absolutely not. > __packed tells the compiler that the structure might be on any address boundary. > On many architectures this means the compiler must use byte accesses with shifts > and ors for every access. > The most a hardware defined structure will have is a compile-time assert > that it is the correct size (to avoid silly errors from changes). Ok - interesting, I see what you're saying - and certainly don't want the compiler to be performing byte accesses on the structures unnecessarily. I'm trying to distinguish the difference here. Is the single point that __packed causes byte-access, where as __attribute__((__packed__)); does not? Looking at the GCC docs [0]: I see that __attribute__((__packed__)) tells the compiler that the "structure or union is placed to minimize the memory required". However, the keil compiler docs[1] do certainly declare that __packed causes byte alignment. I was confused/thrown off here by the Kernel defining __packed as __attribute__((packed)) at [2]. Do __attribute__((packed)) and __attribute__((__packed__)) differ ? In which case, from what I've read so far I wish "__packed" was "__unaligned"... [0] https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-packed-type-attribute [1] http://www.keil.com/support/man/docs/armcc/armcc_chr1359124230195.htm [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/compiler-gcc.h?h=v4.16-rc5#n92 Regards Kieran > David >
