Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removingthe packed attribute

From: Alexander Holler
Date: Thu Jun 16 2011 - 16:11:23 EST

Am 16.06.2011 21:46, schrieb Alan Stern:
On Thu, 16 Jun 2011, Alexander Holler wrote:

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?

We're talking about padding, not alignment. Obviously these two
concepts are related, since fields with differing alignment
requirements sometimes force the compiler to insert padding. But they
aren't the same thing.

The question is whether gcc will insert padding in a structure that
doesn't need it. The kernel depends on peculiarities of gcc in many
ways; basing your strategy on what the C99 spec says is not always a
good idea.
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).

Hmmm. I won't say that ((packed,aligned(4))) is wrong. But it's not
clearly necessary either.

Maybe I should have added a "cosmetic:" in front of the subject of the patch. ;)

Using packed doesn't seem to be necessary (at least not with those versions of gcc I'm using here), I've tried both versions (on arm, without packed and with packed, aligned(4)) and both are working. I've only posted the patch because I found the usage of packed, aligned(4) much clearer than without packed. And It might help avoiding such discussions like this with people like me who aren't that deep involved in gcc-specific implementation details. ;)

Anyway, feel free to nack that patch. I don't really care and just thought I should post it (e.g. as an alternative to removing that packed).


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