Re: [PATCH 02/11] media: vsp1: use kernel __packed for structures

[Kieran Bingham]

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
>