arm/155214: [patch] MMC/SD IO slow on Atmel ARM with modern large SD cards

[Bernd Walter]

On Fri, Mar 04, 2011 at 08:20:11PM +0000, Ian Lepore wrote:
> The following reply was made to PR arm/155214; it has been noted by GNATS.
> 
> From: Ian Lepore <freebsd at damnhippie.dyndns.org>
> To: ticso at cicely.de
> Cc: FreeBSD-gnats-submit at freebsd.org
> Subject: Re: arm/155214: [patch] MMC/SD IO slow on Atmel ARM with modern
>  large SD cards
> Date: Fri, 04 Mar 2011 13:10:12 -0700
> 
>  On Thu, 2011-03-03 at 00:52 +0100, Bernd Walter wrote:
>  > On Wed, Mar 02, 2011 at 02:53:18PM -0700, Ian Lepore wrote:
>  Since 15mhz/4bit is still twice the data throughput of 30mhz/1bit I 
>  decided to do some crude benchmarking to see if it's worth the trouble 
>  of making 4-bit work correctly.  The results appear below.  In 
>  summary, there is definitely a benefit to using 4-bit transfers, but 
>  the improvement isn't nearly as dramatic as the change from single- to 
>  multi-block IO.  

4bit transport has a more interesting point than bandwidth.
An 64k read with 15MHz/1bit will have ~35ms calculated latency just
for the data transport, which I consider a pretty high value.
Nevertheless I'm still exited about your multiblock support and your
measured values already look fantastic.

>  Based on the benchmark results, and the fact that I don't really have 
>  the time to take on the dev/mmc changes right now, I think we should 
>  adopt the multi-block patches and stick with 30mhz/1-bit for now.  

I completely agree with you.
The main problem was write performance and this is clearly solved to
match with media capabilities - not to speak that the former single
block writes also had an increased media wear.

-- 
B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.