Re: [PATCH v2 04/15] ACPI: table: replace __attribute__((packed)) by __packed

[Rafael J. Wysocki]

On Thu, Apr 1, 2021 at 11:00 AM David Laight <David.Laight@xxxxxxxxxx> 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@xxxxxxxxx>
> > > 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.