Re: remove block layer bounce buffering for MMC

From: Linus Walleij
Date: Wed Jan 16 2019 - 08:51:33 EST

On Wed, Jan 16, 2019 at 11:25 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Mon, Jan 14, 2019 at 11:27 AM Ulf Hansson <ulf.hansson@xxxxxxxxxx> 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.

Linus Walleij