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

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Message-ID: <CAK8P3a16ZoBt=HpZByb_1iH+-0e9O0F0sPX+ROfO9hWnJR94qg@mail.gmail.com>
Date:   Wed, 16 Jan 2019 11:24:43 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Christoph Hellwig <hch@....de>,
        Linus Walleij <linus.walleij@...aro.org>,
        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 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).
>
> On Mon, 14 Jan 2019 at 10:58, Christoph Hellwig <hch@....de> wrote:
> >
> > Hi everyone,
> >
> > this series converts the remaining MMC host drivers to properly kmap the
> > scatterlist entries it does PIO operations on, and then goes on to
> > remove the usage of block layer bounce buffering (which I plan to remove
> > eventually) from the MMC layer.
> >
> > As a bonus I've converted various drivers to the proper scatterlist
> > helpers so that at least in theory they are ready for chained
> > scatterlists.
> >
> > All the changes are compile tested only as I don't have any of the
> > hardware, so a careful review would be appreciated.
>
> Thanks for posting this. I will have a look as soon as I can, but
> first needs to catch up with things since the holidays.

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.

       Arnd