From: Catalin Marinas <catalin.marinas@arm.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Ard Biesheuvel <ardb@kernel.org>,
Vincent Whitchurch <vincent.whitchurch@axis.com>,
Will Deacon <will@kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
kernel@axis.com
Subject: Re: [PATCH] arm64: Make ARCH_DMA_MINALIGN configurable
Date: Mon, 17 May 2021 15:20:25 +0100 [thread overview]
Message-ID: <20210517142025.GD1106@arm.com> (raw)
In-Reply-To: <CAK8P3a17UyHiNoHKu4d_Uyi+jmbpprN+vNkqb53MMHbJTO2htw@mail.gmail.com>
On Mon, May 17, 2021 at 03:35:39PM +0200, Arnd Bergmann wrote:
> On Mon, May 17, 2021 at 2:01 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> > On Mon, 17 May 2021 at 13:06, Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > On Mon, May 17, 2021 at 09:43:32AM +0200, Vincent Whitchurch wrote:
> > >
> > > Another option I recall discussing with Arnd about two years ago was to
> > > start with the default 128 at boot but add the smaller slab caches
> > > later, once we have more information. This can be just another 64 byte
> > > cache or even go all the way down to 8 byte if all the devices are
> > > cache coherent.
> > >
> >
> > ARCH_SLAB_MINALIGN is also used to statically align (members of)
> > struct types, so doing this at runtime is going to have limited
> > effect.
> >
> > If a) ThunderX is the only platform we care about (do we?) that has
> > 128 byte cachelines, and b) DMA is cache coherent on such platforms,
> > couldn't we separate ARCH_SLAB_MINALIGN from ARCH_DMA_MINALIGN? I.e.,
> > set the first to 64 and keep the second at 128?
>
> What is the purpose of ARCH_DMA_MINALIGN then? If it's cache
> coherent, does DMA buffer still need to be aligned to the cache line
> size?
For coherent DMA, we don't need the alignment from a correctness
perspective.
For performance, a driver can always pass SLAB_HWCACHE_ALIGN which ends
up using cache_line_size(), so it gets the actual hardware value.
--
Catalin
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next prev parent reply other threads:[~2021-05-17 14:24 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-17 7:43 [PATCH] arm64: Make ARCH_DMA_MINALIGN configurable Vincent Whitchurch
2021-05-17 11:04 ` Catalin Marinas
2021-05-17 12:01 ` Ard Biesheuvel
2021-05-17 13:35 ` Arnd Bergmann
2021-05-17 14:20 ` Catalin Marinas [this message]
2021-05-17 14:15 ` Catalin Marinas
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