From: Dexuan Cui <de...@microsoft.com>
Date: Fri, 9 Apr 2021 00:24:51 +0000
>> From: David Miller <da...@davemloft.net>
>> Sent: Thursday, April 8, 2021 4:46 PM
>> ...
>> > +struct gdma_msg_hdr {
>> > + u32 hdr_type;
>> > + u32 msg_type;
>> > + u16 msg_version;
>> > + u16 hwc_msg_id;
>> > + u32 msg_size;
>> > +} __packed;
>> > +
>> > +struct gdma_dev_id {
>> > + union {
>> > + struct {
>> > + u16 type;
>> > + u16 instance;
>> > + };
>> > +
>> > + u32 as_uint32;
>> > + };
>> > +} __packed;
>>
>> Please don't use __packed unless absolutely necessary. It generates
>> suboptimal code (byte at a time
>> accesses etc.) and for many of these you don't even need it.
>
> In the driver code, all the structs/unions marked by __packed are used to
> talk with the hardware, so I think __packed is necessary here?
It actually isan't in many cases, check with and without the __packed directive
and see if anything chasnges.
> Do you think if it's better if we remove all the __packed, and add
> static_assert(sizeof(struct XXX) == YYY) instead? e.g.
>
> @@ -105,7 +105,8 @@ struct gdma_msg_hdr {
> u16 msg_version;
> u16 hwc_msg_id;
> u32 msg_size;
> -} __packed;
> +};
> +static_assert(sizeof(struct gdma_msg_hdr) == 16);
This won't make sure the structure member offsets are what you expect.
I think you'll have to go through the structures one-by-one by hand to
figure out which ones really require the __packed attribute and which do not.
Thank you.
