linux-kernel - Re: remove block layer bounce buffering for MMC

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Date:   Wed, 16 Jan 2019 14:51:16 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Christoph Hellwig <hch@....de>,
        Russell King <linux@...linux.org.uk>,
        Nicolas Pitre <nico@...xnic.net>,
        Aaro Koskinen <aaro.koskinen@....fi>,
        Ben Dooks <ben-linux@...ff.org>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-omap <linux-omap@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: remove block layer bounce buffering for MMC

On Wed, Jan 16, 2019 at 11:25 AM Arnd Bergmann <arnd@...db.de> wrote:
> On Mon, Jan 14, 2019 at 11:27 AM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >
> > +Linus Walleij (recently made a cleanup of the mmc bounce buffering code).

Nah it's not THAT bounce buffer.

> Linus probably knows more here, but I have a vague recollection of
> the MMC bounce buffer code being needed mostly for performance
> reasons: when the scatterlist is discontiguous, that can result in
> a request being split up into separate MMC commands, which due
> to the lack of queued commands combined with the need for
> garbage collection on sub-page writes results in a huge slowdown
> compared to having larger bounce buffers all the time.
>
> We had discussed finding a different way to do this (separate
> from the bounce buffering), but I don't know if that ever happened,
> or if this is even the code that you are changing here.

Nope not the same code.

The term "bounce buffer" is sadly used as ambigously as
__underscores in front of function names.

That other "bounce buffer" was first deleted and then
reimplemented as a local hack in the SDHCI driver core
after it caused performance regressions on the i.MX and
some laptops, see commit:

commit bd9b902798ab14d19ca116b10bde581ddff8f905
mmc: sdhci: Implement an SDHCI-specific bounce buffer

That should be orthogonal to Christoph's changes in this
patch series.

Yours,
Linus Walleij