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. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
