On Sat, 26 Jan 2008, David Miller wrote:
>
> PowerPC handles unaligned accesses transparently, so it is safe
> for them to make this setting.
.. as does x86. Better than PowerPC, btw.
> It would not work out on sparc64 for example.
Right. Which is why different architectures should do different things,
and which is why this should absolutely NOT be a driver issue. The driver
should do what is natural to it.
If giving aligned data is reasonable, then obviously the driver should be
striving for that, since regardless of architecture, alignment will never
HURT the CPU.
But you seem to be totally ignoring people when they tell you that the
hardware cannot necessarily do unaligned DMA well, or at all.
We've had that with disk controllers that simply *cannot* do DMA at less
than 4-byte alignment, and where anything else has to be either done in
software with PIO, or by memcpy.
You've also had Intel engineers tell you that the actual wireless stuff
isn't just "firmware" - everybody who claims that this is an easy fix from
Intel apparently has just been outright *lying*.
> So you'll see drivers for chips that have to 64-byte align the
> ethernet header in the RX packet for whatever reason, simply
> copy into a fresh buffer on receive.
>
> Some drivers do this only platforms where unalign accesses trap or
> simply do not work.
.. and can't you see that this is STUPID?
Honestly, now, David - take a step back, and allow yourself to think about
the issue. You have the following undeniable FACTS:
- some hardware only does certain DMA alignment. This is a FACT. Treat it
as such, instead of whining about how Intel alledgely could easily fix
it.
Quite frankly, I've been disgusted by people who say that it's "easy to
fix firmware". It's not. It then turns out that Intel people actually
step up and tell you that it's not even a firmware issue, are you now
going to say "It's easy to fix hardware - just a few lines of RTL"?
So instead of this idiotic whining about "easy to fix firmware" that
turned out to be just total ignorance, how about the people who made
that claim step up and admit that (a) apparently not true, but also (b)
even if it had been "just" firmware, that was a totally idiotic claim
to begin with, since it's never been a valid excuse anyway!
And once you admit that first part ("some hardware will have different
aligment requirements than the network stack has"), go on to the OBVIOUS
secont step, which is to realize two other undeniable truths:
- requiring individual drivers to do something in every driver is stupid
if it all boils down to something that could be done in *one* place
(the network stack entry).
This is so obviously true that I really cannot see why you don't seem
to accept this. Drivers are hard enough to write as-is, and we want to
make driver writers have to worry as little as possible about things
like this.
- The above is *doubly* true when it's simply not the case that "drivers
should always align packets to X bytes".
Many CPU's handle unaligned accesses with little or no cost at all.
Most x86 chips have absolutely *zero* cost for an unaigned access that
doesn't cross a cache access boundary (often 8 bytes, these days I
think it's 16), and even when it requires crossing the natural internal
cache access bus width, it's usually not that high.
In other words, what you ask drivers to do is to not just create extra
work for them, but create extra work for them in ways that aren't even
obvious or relevant to most driver writers. Because on many architectures,
they are better off *not* aligining things.
End result? When Intel writes a driver, they do it for the CPU
architecture they care about. Which does *not* have the idiotic unaligned
access problems that sparc has. So they do what makes sense to them, and I
reall ycannot blame them for it.
So there's a very simple solution to this all: just make the entry to the
networking layer re-align thing if required. In *one* single well-tested
place. One that can do some smart things, like decide that re-aligning
small packets may make sense even on architectures that don't have
problems with unaligned accesses, just because the packet fits in one
cacheline anyway.
That way al drivers can do what is best for *them*, and not have to worry.
Yes, they'd like to align things naturally (in order to avoid the cost of
the memcp()), but you don't create unnecessary and *stupid* work for them.
Linus
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