Messages in this thread | | | Date | Thu, 16 Jun 2011 21:25:40 +0200 | From | Alexander Holler <> | Subject | Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removing the packed attribute |
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Am 16.06.2011 19:55, schrieb Arnd Bergmann: > On Thursday 16 June 2011, Alan Stern wrote: >> >> On Thu, 16 Jun 2011, Alexander Holler wrote: >> >>> In commit 139540170d9d9b7ead3caaf540f161756b356d56 the attribute >>> packed is removed from the structs which are used to access the EHCI-registers. >>> >>> This is done to circumvent a problem with gcc 4.6, which might access members of >>> packed structs on a byte by byte basis. But using packed, aligned(4) fixes that >>> too and is imho the better solution. Otherwise (without packed) the compiler would be free >>> to choose whatever alignment he thinks fits best, which might be e.g. 8-byte on 64-bit machines. >> >> Is that really true? > > No. > >> I thought the compiler was not allowed to insert >> padding if the natural alignment of the data types didn't require any. > > It's architecture dependent. The alignment of the structure is the maximum alignment > of its members, so it gets to be 8 bytes if there is a 64 bit member in the struct > on most architectures, but 4 bytes on x86.
Hmm, sorry, but that sentence just says something about the alignment of the structure itself and nothing about the alignment of it's members or do I understand something wrong?
I've had a look at c99 again, and in addition to the two points in c99 I mentioned in the mail before (6.7.2.1 13 and 6.7.2.1. 15), I've only found the following on that topic:
6.7.2.1 12 Each non-bit-field member of a structure or union object is aligned in an implementationdefined manner appropriate to its type.
And, under "J.1 Unspecified behaviour":
Many aspects of the representations of types (6.2.6).
I even haven't found anything which says something about the alignment of a structure itself. But I'm no compiler expert and I look only seldom at c99 and usually try to avoid such aspects as the one we are talking about. ;)
For me that means that I understand that when packed(,aligned(4)) is used, it's pretty sure, that there is no padding inbetween the members of e.g. struct ehci_regs. But without I'm unsure, so I would avoid that.
That aligned(4) is necessary (for ARM) is only a workaround because of the implementation of readl(), at least that is how I understood the discussion. But that is discussed elsewhere and don't want to take part in that discussion (and can't).
Regards,
Alexander
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