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

[Warner Losh]

On 03/02/2011 17:10, Bernd Walter wrote:
> The following reply was made to PR arm/155214; it has been noted by GNATS.
>
> From: Bernd Walter<ticso at cicely7.cicely.de>
> To: Ian Lepore<freebsd at damnhippie.dyndns.org>
> 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: Thu, 3 Mar 2011 00:52:51 +0100
>
>   On Wed, Mar 02, 2011 at 02:53:18PM -0700, Ian Lepore wrote:
>   >
>   >  >Number:         155214
>   >  >Category:       arm
>   >  >Synopsis:       [patch] MMC/SD IO slow on Atmel ARM with modern large SD cards
>   >  >Confidential:   no
>   >  >Severity:       serious
>   >  >Priority:       medium
>   >  >Responsible:    freebsd-arm
>   >  >State:          open
>   >  >Quarter:
>   >  >Keywords:
>   >  >Date-Required:
>   >  >Class:          sw-bug
>   >  >Submitter-Id:   current-users
>   >  >Arrival-Date:   Wed Mar 02 22:10:10 UTC 2011
>   >  >Closed-Date:
>   >  >Last-Modified:
>   >  >Originator:     Ian Lepore<freebsd at damnhippie.dyndns.org>
>   >  >Release:        FreeBSD 8.2-RC3 arm
>   >  >Organization:
>   >  none
>   >  >Environment:
>   >  FreeBSD dvb 8.2-RC3 FreeBSD 8.2-RC3 #49: Tue Feb 15 22:52:14 UTC 2011     root at revolution.hippie.lan:/usr/obj/arm/usr/src/sys/DVB  arm
>   >
>   >  Included patch is against -current even though the problem was first seen on
>   >  8.2-RC3
>   >
>   >  The problem was seen on AT91RM9200 hardware, but presumably also affects the
>   >  SAM9 series which uses the same driver code.
>   >
>   >  >Description:
>   >  With the latest generation of large-capacity SD cards, write speeds as low as
>   >  20 kbytes/sec are seen.  These modern cards have erase-block sizes as large as
>   >  8192K (compared to 32K typical on previous generations).  The at91_mci driver
>   >  does only single-sector IO; apparently this requires the SD card to internally
>   >  perform an expensive read-erase-modify-write cycle for each 512 byte block
>   >  written to the card.
>
>   The complete details of this problem are completely known.
>   However the RM9200 has many hardware problems to be worked around and
>   so far noone actually did.
>   Your patch is quite large, so I would like to ask you explicitly:
>   Did you test your patch with an AT91RM9200 system?

I believe their only playform is AT91RM9200 based...

>   You did enable multisector support for reading and (more important) for
>   writing?
>   But you didn't activate 4bit mode?
>   With 4bit mode there is no hardware bug, but when the driver was written
>   is was just done in a lazy way because activating 4bit on SD cards require
>   special handling - in the meantime the SD layer itself was extracted and
>   has 4bit support, but the at91_mci driver was never updated to use that.

Actually, I was never able to make the 4-bit mode work because our 
primary hardware platfom had only 1 bit and the eval boards that I 
bought that had 4-bits wired up would never work completely reliably.  
Since I was never able to test it reliably, I never completed the 
partial implementation I did.

>   PS: I'm very pleased to see your work since SD write speed was a
>   major show stopper for some applications

Agreed.  This stuff is very cool.

Warner