Date: Mon, 27 Aug 2012 08:20:52 -0700 From: Adrian Chadd <adrian@freebsd.org> To: Tim Kientzle <tim@kientzle.com> Cc: Hans Petter Selasky <hans.petter.selasky@bitfrost.no>, freebsd-arm@freebsd.org, freebsd-mips@freebsd.org, freebsd-arch@freebsd.org Subject: Re: Partial cacheline flush problems on ARM and MIPS Message-ID: <CAJ-VmomMCvR7nr73sLLmwsotU=TT399asBcJvinHCqspo6BN1w@mail.gmail.com> In-Reply-To: <DA9750F9-7B8A-49AF-8ECA-AC7D565CF3F5@kientzle.com> References: <6D83AF9D-577B-4C83-84B7-C4E3B32695FC@bsdimp.com> <zarafa.503b0e81.5c36.1a2f71091ebf9bd2@eric2.bitfrost> <A749E691-BF25-4B72-B929-56ABEB10F3E9@bsdimp.com> <DA9750F9-7B8A-49AF-8ECA-AC7D565CF3F5@kientzle.com>
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On 27 August 2012 08:12, Tim Kientzle <tim@kientzle.com> wrote:
>> No. I don't think so. the reason is that you can't define USB_DMA_ALGIN to be a constant on MIPS, at least, or I think ARM because that's determined at run time.
FYI, I wonder if MIPS may need some run time value for that. Right
now we hide it behind needing a custom config file for each mips
variant, rather than supporting one mips24k kernel for ${LOTS} of
variants.
> But don't mbuf structures do pretty much what Hans is suggesting?
>
> Why is mbuf okay?
Which part of mbufs?
I think the difference here is that there's no concurrent access. Ie,
although you may have an mbuf header + data inside the same cache
line, you wouldn't go fondling the mbuf once it's handed to the
hardware.
But I was under the impression that mbuf + mbuf buffers are already
correctly aligned.
This all honestly looks like a very i386-centric interpretation of the
busdma "intention", which I think illustrates a need to better
document what assumptions busdma actually makes.
That does remind me, I think the ath(4) driver does the same (since it
allocates its own descriptor block and then treats it as an array of
descriptors for the hardware to access) - I should ensure that
sizeof(ath_desc) is aligned on the relevant architecture. It gets
slightly scary - AR93xx TX descriptors are "L1 cache == 128 byte
aligned" which is an enormous waste of memory compared to a 16 or 32
byte aligned platform. Alas..
Adrian
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