linux-kernel - RE: [PATCH v2 04/15] ACPI: table: replace __attribute__((packed)) by _

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Date:   Mon, 29 Mar 2021 10:09:13 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Xiaofei Tan' <tanxiaofei@...wei.com>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        "lenb@...nel.org" <lenb@...nel.org>,
        "rui.zhang@...el.com" <rui.zhang@...el.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>
CC:     "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linuxarm@...neuler.org" <linuxarm@...neuler.org>
Subject: RE: [PATCH v2 04/15] ACPI: table: replace __attribute__((packed)) by
 __packed

From: Xiaofei Tan
> Sent: 27 March 2021 07:46
> 
> Replace __attribute__((packed)) by __packed following the
> advice of checkpatch.pl.
> 
> Signed-off-by: Xiaofei Tan <tanxiaofei@...wei.com>
> ---
>  drivers/acpi/acpi_fpdt.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/acpi/acpi_fpdt.c b/drivers/acpi/acpi_fpdt.c
> index a89a806..690a88a 100644
> --- a/drivers/acpi/acpi_fpdt.c
> +++ b/drivers/acpi/acpi_fpdt.c
> @@ -53,7 +53,7 @@ struct resume_performance_record {
>  	u32 resume_count;
>  	u64 resume_prev;
>  	u64 resume_avg;
> -} __attribute__((packed));
> +} __packed;
> 
>  struct boot_performance_record {
>  	struct fpdt_record_header header;
> @@ -63,13 +63,13 @@ struct boot_performance_record {
>  	u64 bootloader_launch;
>  	u64 exitbootservice_start;
>  	u64 exitbootservice_end;
> -} __attribute__((packed));
> +} __packed;
> 
>  struct suspend_performance_record {
>  	struct fpdt_record_header header;
>  	u64 suspend_start;
>  	u64 suspend_end;
> -} __attribute__((packed));
> +} __packed;

My standard question about 'packed' is whether it is actually needed.
It should only be used if the structures might be misaligned in memory.
If the only problem is that a 64bit item needs to be 32bit aligned
then a suitable type should be used for those specific fields.

Those all look very dubious - the standard header isn't packed
so everything must eb assumed to be at least 32bit aligned.

There are also other sub-structures that contain 64bit values.
These don't contain padding - but that requires 64bit alignement.

The only problematic structure is the last one - which would have
a 32bit pad after the header.
Is this even right given than there are explicit alignment pads
in some of the other structures.

If 64bit alignment isn't guaranteed then a '64bit aligned to 32bit'
type should be used for the u64 fields.

	David

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