[PATCH RFC 0/3] mmc: mxs-mmc: Implement DDR support

[Stefan Wahren]

Hi Marek,

> Marek Vasut <marex at denx.de> hat am 6. August 2016 um 15:10 geschrieben:
> 
> 
> On 08/06/2016 02:55 PM, Stefan Wahren wrote:
> > Based on these discussions: [1], [2] this patch series implements
> > DDR support for the MXS MMC host driver. This feature has never been
> > ported from the vendor kernel.
> > 
> > It has been tested on a i.MX28 board with eMMC which is currently
> > not in mainline (Duckbill 2).
> > 
> > * without DDR support
> > dd if=/dev/zero of=test bs=8k count=10000
> > 81920000 bytes (82 MB) copied, 14.3321 s, 5.7 MB/s
> > 
> > * with DDR support:
> > dd if=/dev/zero of=test bs=8k count=10000
> > 81920000 bytes (82 MB) copied, 13.4781 s, 6.1 MB/s
> 
> Not much of a speedup, how so ?

i agree. Unfortunately i never had the time for a deeper analysis, but i noticed
big write performance differences between the vendor kernel 2.6.35 and current
mainline kernel in case of writing 100 MB directly to an eMMC partition (factor
2). Implementation of DDR mode was the lower fruit.

I've the suspicion that is related to the usage of DMA engine. Maybe the
overhead for AC/BC commands is to big. In the Freescale kernel is an
optimization [1].

[1] -
http://git.freescale.com/git/cgit.cgi/imx/linux-2.6-imx.git/commit/?h=imx_2.6.35_maintain&id=b09358887fb4b67f6d497fac8cc48475c8bd292d

> 
> > [1] - http://www.spinics.net/lists/linux-mmc/msg32285.html
> > [2] - http://www.spinics.net/lists/linux-mmc/msg34418.html
> > 
> > Stefan Wahren (3):
> >   DT: bindings: mmc: Add property for 3.3V only support
> >   mmc: core: add new cap for 3.3V only DDR MMCs
> >   mmc: mxs-mmc: Implement DDR support
> > 
> >  Documentation/devicetree/bindings/mmc/mmc.txt |    7 +++++--
> >  drivers/mmc/core/host.c                       |    2 ++
> >  drivers/mmc/core/mmc.c                        |    6 ++++++
> >  drivers/mmc/host/mxs-mmc.c                    |   22 ++++++++++++++++++++++
> >  include/linux/mmc/host.h                      |    1 +
> >  5 files changed, 36 insertions(+), 2 deletions(-)
> > 
> 
> 
> -- 
> Best regards,
> Marek Vasut