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

[Greg Ansley]

On 3/2/11 7:40 PM, 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: Wed, 02 Mar 2011 17:21:09 -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:
>   >  >
>   >  >  >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?
>   >  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.
>   >
>   >  PS: I'm very pleased to see your work since SD write speed was a
>   >  major show stopper for some applications
>   >
>
>   Yes, the patch is large, partly because I included comments about the
>   hardware problems I found and how the code works around them (and also
>   to help the next person understand the flow).
>
>   My changes support multi-sector IO for both reads and writes.
>
>   The company I work for uses the AT91RM9200 on custom-designed boards in
>   8 products, all with substantially similar board designs.  So far we've
>   tested these changes on 4 of them, with no problems found.
>
>   I have not tested with 4-bit enabled; I wasn't aware (but in retrospect
>   I probably should have assumed) that the hardware bugs are different
>   with 4-bit enabled.  I'm not even sure our hardware design carries all 4
>   lines to the card; I'll look at the schematics and if they're connected
>   I'll see about testing that mode.  (And if they're not I'll see about
>   having our designers wire up all 4 lines on future designs.)
>
>   I also haven't tested with the SAM9-series, because I don't have that
>   hardware available.  (I hope to convince our hardware designers to
>   migrate us to SAM9 this year.)
>
With the current code (prepatch) 4bit mode is known to work at least on 
the SAM9G20 with kernel option AT91_MCI_HAS_4WIRE.  I'll be working on 
the SAM9G20 in the next few days and I can test the patch on both a 
RM9200 (1 bit only) and on a couple of SAM9G20 designs with 4bit hardware.

Greg