From: "Rafael J. Wysocki" <rafael@kernel.org>
To: David Laight <David.Laight@aculab.com>
Cc: Bjorn Helgaas <helgaas@kernel.org>,
Zhang Rui <rui.zhang@intel.com>,
Xiaofei Tan <tanxiaofei@huawei.com>,
"rjw@rjwysocki.net" <rjw@rjwysocki.net>,
"lenb@kernel.org" <lenb@kernel.org>,
"bhelgaas@google.com" <bhelgaas@google.com>,
"linux-acpi@vger.kernel.org" <linux-acpi@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
"linuxarm@openeuler.org" <linuxarm@openeuler.org>
Subject: Re: [PATCH v2 04/15] ACPI: table: replace __attribute__((packed)) by __packed
Date: Thu, 1 Apr 2021 15:49:52 +0200 [thread overview]
Message-ID: <CAJZ5v0jJ2XDYSwqP3AyKuUvuxhwuNwvk3Z=xwtAL3hG5uYGG-Q@mail.gmail.com> (raw)
In-Reply-To: <100f5a45dae14c77b341b7f1c5ea1db0@AcuMS.aculab.com>
On Thu, Apr 1, 2021 at 11:00 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Bjorn Helgaas
> > Sent: 31 March 2021 18:22
> >
> > On Wed, Mar 31, 2021 at 11:55:08PM +0800, Zhang Rui wrote:
> > > ...
> >
> > > From e18c942855e2f51e814d057fff4dd951cd0d0907 Mon Sep 17 00:00:00 2001
> > > From: Zhang Rui <rui.zhang@intel.com>
> > > Date: Wed, 31 Mar 2021 20:34:13 +0800
> > > Subject: [PATCH] ACPI: tables: FPDT: Fix 64bit alignment issue
> > >
> > > Some of the 64bit items in FPDT table may be 32bit aligned.
> > > Using __attribute__((packed)) is not needed in this case, fixing it by
> > > allowing 32bit alignment for these 64bit items.
> >
> > 1) Can you please add a spec reference for this? I think it's ACPI
> > v6.3, sec 5.2.23.5, or something close to that.
> >
> > 2) The exact layout in memory is prescribed by the spec. I think
> > that's basically what "packed" accomplishes. I don't understand
> > why using "aligned" would be preferable. Using "aligned" means
> > things can be at different offsets depending on the starting
> > address of the structure. We always want the identical layout, no
> > matter what the starting address is.
>
> Both 'packed' and 'aligned(4)' remove any structure alignment
> padding before 64bit items that aren't on an 8 byte boundary.
> (Because everything else in the structures is naturally aligned.)
>
> The difference is significant on cpu that don't support misaligned
> addresses.
> Assuming that the structure is always on a 4n byte boundary
> (which the ACPI spec probably requires) accesses to the 32-bit
> fields are always ok.
> It is only 64-bit fields that must be accessed as two 32-bit
> memory cycles, not all the fields using multiple single byte
> cycles.
So what exactly is wrong with using "packed"? It is way easier to
understand for a casual reader of the code.
next prev parent reply other threads:[~2021-04-01 17:56 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-27 7:46 [PATCH v2 00/15] acpi: fix some coding style issues Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 01/15] ACPI: APD: fix a block comment align issue Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 02/15] ACPI: processor: fix some coding style issues Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 03/15] ACPI: debug: " Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 04/15] ACPI: table: replace __attribute__((packed)) by __packed Xiaofei Tan
2021-03-29 10:09 ` David Laight
2021-03-30 2:23 ` Xiaofei Tan
2021-03-30 7:31 ` Zhang Rui
2021-03-30 7:59 ` Zhang Rui
2021-03-30 8:14 ` David Laight
2021-03-30 17:00 ` Rafael J. Wysocki
2021-03-31 15:55 ` Zhang Rui
2021-03-31 16:14 ` David Laight
2021-03-31 17:22 ` Bjorn Helgaas
2021-04-01 8:59 ` David Laight
2021-04-01 13:49 ` Rafael J. Wysocki [this message]
2021-04-01 14:23 ` David Laight
2021-04-01 17:29 ` Rafael J. Wysocki
2021-03-27 7:46 ` [PATCH v2 05/15] ACPI: ipmi: remove useless return statement for void function Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 06/15] ACPI: LPSS: fix some coding style issues Xiaofei Tan
[not found] ` <CAHp75Vc+0hxLS_Ab7_VZfrG2jiQzvia9S=o+Gc+wg+vVk1Z39w@mail.gmail.com>
2021-03-27 9:55 ` Xiaofei Tan
[not found] ` <CAHp75Vd0hVqsfsZK=d1dz98Kbchqz-w4RqQQp6FwisxSGG5BzA@mail.gmail.com>
2021-03-27 9:58 ` Xiaofei Tan
2021-03-27 13:39 ` Joe Perches
2021-03-27 16:39 ` Andy Shevchenko
2021-03-27 7:46 ` [PATCH v2 07/15] ACPI: memhotplug: fix a coding style issue Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 08/15] ACPI: acpi_pad: " Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 09/15] ACPI: battery: fix some coding style issues Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 10/15] ACPI: button: " Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 11/15] ACPI: CPPC: " Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 12/15] ACPI: custom_method: fix a coding style issue Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 13/15] ACPI: PM: fix some coding style issues Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 14/15] ACPI: sysfs: " Xiaofei Tan
2021-03-27 7:46 ` [PATCH v2 15/15] ACPI: dock: " Xiaofei Tan
[not found] ` <CAHp75VcwuFYWRYfVPxbqa4TFGgqOhHc_soefmTAZcGGk3bLuhw@mail.gmail.com>
2021-03-27 10:00 ` [PATCH 00/15] acpi: " Xiaofei Tan
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