The bounce buffer is gone from the MMC core, and now we found out
that there are some (crippled) i.MX boards out there that have broken
ADMA (cannot do scatter-gather), and also broken PIO so they must
use SDMA. Closer examination shows a less significant slowdown
also on SDMA-only capable Laptop hosts.

SDMA sets down the number of segments to one, so that each segment
gets turned into a singular request that ping-pongs to the block
layer before the next request/segment is issued.

Apparently it happens a lot that the block layer send requests
that include a lot of physically discontiguous segments. My guess
is that this phenomenon is coming from the file system.

These devices that cannot handle scatterlists in hardware can see
major benefits from a DMA-contiguous bounce buffer.

This patch accumulates those fragmented scatterlists in a physically
contiguous bounce buffer so that we can issue bigger DMA data chunks
to/from the card.

When tested with a PCI-integrated host (1217:8221) that
only supports SDMA:
0b:00.0 SD Host controller: O2 Micro, Inc. OZ600FJ0/OZ900FJ0/OZ600FJS
        SD/MMC Card Reader Controller (rev 05)
This patch gave ~1Mbyte/s improved throughput on large reads and
writes when testing using iozone than without the patch.

dmesg:
sdhci-pci 0000:0b:00.0: SDHCI controller found [1217:8221] (rev 5)
mmc0 bounce up to 128 segments into one, max segment size 65536 bytes
mmc0: SDHCI controller on PCI [0000:0b:00.0] using DMA

On the i.MX SDHCI controllers on the crippled i.MX 25 and i.MX 35
the patch restores the performance to what it was before we removed
the bounce buffers.

Cc: Pierre Ossman <pierre@ossman.eu>
Cc: Benoît Thébaudeau <benoit@wsystem.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Benjamin Beckmeyer <beckmeyer.b@rittal.de>
Cc: stable@vger.kernel.org # v4.14+
Fixes: de3ee99b ("mmc: Delete bounce buffer handling")
Tested-by: Benjamin Beckmeyer <beckmeyer.b@rittal.de>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>