On Tue, Mar 30, 2021 at 10:15 AM David Laight David.Laight@aculab.com wrote:
From: Zhang Rui
Sent: 30 March 2021 09:00 To: Xiaofei Tan tanxiaofei@huawei.com; David Laight David.Laight@ACULAB.COM; rjw@rjwysocki.net; lenb@kernel.org; bhelgaas@google.com Cc: linux-acpi@vger.kernel.org; linux-kernel@vger.kernel.org; linux-pci@vger.kernel.org; linuxarm@openeuler.org Subject: Re: [PATCH v2 04/15] ACPI: table: replace __attribute__((packed)) by __packed
On Tue, 2021-03-30 at 15:31 +0800, Zhang Rui wrote:
On Tue, 2021-03-30 at 10:23 +0800, Xiaofei Tan wrote:
Hi David,
On 2021/3/29 18:09, David Laight wrote:
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@huawei.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.
Yes, some of them has been aligned already, then nothing changed when add this "packed ". Maybe the purpose of the original author is for extension, and can tell others that this struct need be packed.
The patch is upstreamed recently but it was made long time ago. I think the original problem is that one of the address, probably the suspend_performance record, is not 64bit aligned, thus we can not read the proper content of suspend_start and suspend_end, mapped from physical memory.
I will try to find a machine to reproduce the problem with all __attribute__((packed)) removed to double confirm this.
So here is the problem, without __attribute__((packed))
[ 0.858442] suspend_record: 0xffffaad500175020 /sys/firmware/acpi/fpdt/suspend/suspend_end_ns:addr: 0xffffaad500175030, 15998179292659843072 /sys/firmware/acpi/fpdt/suspend/suspend_start_ns:addr: 0xffffaad500175028, 0
suspend_record is mapped to 0xffffaad500175020, and it is combined with one 32bit header and two 64bit fields (suspend_start and suspend_end), this is how it is located in physical memory. So the addresses of the two 64bit fields are actually not 64bit aligned.
David, Is this the "a 64bit item needs to be 32bit aligned" problem you referred? If yes, what is the proper fix? should I used two 32bits for each of the field instead?
Define something like: typedef u64 __attribute__((aligned(4))) u64_align32; and then use it for the 64bit structure members.
There doesn't seem to be a standard type name for it - although it is used in several places.
I'm not entirely sure but is ACPI always LE?
Yes.
(is it even x86 only??)
No.