[PATCH v4 02/11] media: vsp1: Remove packed attributes from aligned structures

[Kieran Bingham]

Hi Laurent,

On 24/05/18 11:47, Laurent Pinchart wrote:
> Hi Kieran,
> 
> Thank you for the patch.
> 
> On Thursday, 3 May 2018 16:36:13 EEST Kieran Bingham wrote:
>> The use of the packed attribute can cause a performance penalty for
>> all accesses to the struct members, as the compiler will assume that the
>> structure has the potential to have an unaligned base.
>>
>> These structures are all correctly aligned and contain no holes, thus
>> the attribute is redundant and negatively impacts performance, so we
>> remove the attributes entirely.
> 
> With gcc 6.4.0 this patch makes no difference on the generated object. Is it 
> worth it ?

This patch has certainly either enlightened me, or confused me about
this topic - as I had in the past used the packed attribute as here to
define that we don't want holes. (just as this existing code does)

As the documentation seems to determine that this isn't the effect of
this attribute, and removing it has no effect - I suspect removing it is
the right thing to do, as otherwise we are simply mis-using the
construct for no purpose.

It's up to though. Drop or accept as you feel is right.

> 
>> Signed-off-by: Kieran Bingham <kieran.bingham+renesas at ideasonboard.com>
> 
> You forget to pick Geert's review tag.

Ah yes, Collected thanks

--
Regards

Kieran


> 
>> ---
>> v2
>>  - Remove attributes entirely
>> ---
>>  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 c7fa1cb088cd..f4cede9b9b43
>> 100644
>> --- a/drivers/media/platform/vsp1/vsp1_dl.c
>> +++ b/drivers/media/platform/vsp1/vsp1_dl.c
>> @@ -25,19 +25,19 @@
>>  struct vsp1_dl_header_list {
>>  	u32 num_bytes;
>>  	u32 addr;
>> -} __attribute__((__packed__));
>> +};
>>
>>  struct vsp1_dl_header {
>>  	u32 num_lists;
>>  	struct vsp1_dl_header_list lists[8];
>>  	u32 next_header;
>>  	u32 flags;
>> -} __attribute__((__packed__));
>> +};
>>
>>  struct vsp1_dl_entry {
>>  	u32 addr;
>>  	u32 data;
>> -} __attribute__((__packed__));
>> +};
>>
>>  /**
>>   * struct vsp1_dl_body - Display list body
>