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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 21 Apr 2022 16:47:24 +0200
Message-ID: <CAK8P3a2Z83TbY6N_TCA-VtE9g6HzP5sySnqqP-D27Uqng3nu9w@mail.gmail.com>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Christoph Hellwig <hch@infradead.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>
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On Thu, Apr 21, 2022 at 4:44 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
> On Thu, Apr 21, 2022 at 03:47:30PM +0200, Arnd Bergmann wrote:
> > On Thu, Apr 21, 2022 at 3:25 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > On Thu, Apr 21, 2022 at 02:28:45PM +0200, Arnd Bergmann wrote:
> > > > We also know that larger slabs are all cacheline aligned, so simply
> > > > comparing the transfer size is enough to rule out most, in this case
> > > > any transfer larger than 96 bytes must come from the kmalloc-128
> > > > or larger cache, so that works like before.
> > >
> > > There's also the case with 128-byte cache lines and kmalloc-192.
> >
> > Sure, but that's much less common, as the few machines with 128 byte
> > cache lines tend to also have cache coherent devices IIRC, so we'd
> > skip the bounce buffer entirely.
>
> Do you know which machines still have 128-byte cache lines _and_
> non-coherent DMA? If there isn't any that matters, I'd reduce
> ARCH_DMA_MINALIGN to 64 now (while trying to get to even smaller kmalloc
> caches).

I think the last time this came up, someone pointed out one of the
Qualcomm Snapdragon phone chips with their custom cores.

        Arnd