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Date:      Mon, 3 Dec 2012 09:17:31 -0700
From:      Warner Losh <imp@bsdimp.com>
To:        "Daisuke Aoyama" <aoyama@peach.ne.jp>
Cc:        freebsd-arm@freebsd.org, Ralf.Wenk@hs-karlsruhe.de
Subject:   Re: FreeBSD on Raspberry Pi 512MB (with U-Boot + ubldr)
Message-ID:  <18DB98C9-66D9-4B00-989A-156F21E9981C@bsdimp.com>
In-Reply-To: <B3D30A45699E443399D7CB112082356B@ad.peach.ne.jp>
References:  <3988C1622A974F19A9D3888F0334FF10@ad.peach.ne.jp> <50B8058C.9030909@bluezbox.com> <E1TfWh4-00BZ82-Is@smtp.hs-karlsruhe.de> <B3D30A45699E443399D7CB112082356B@ad.peach.ne.jp>

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On Dec 3, 2012, at 7:51 AM, Daisuke Aoyama wrote:

>>>> BTW, SDHCI is not yet stable?
>>>=20
>>> There is one issue with setting SDHCI clock. I'll commit fix later =
today.
>>=20
>>=20
>> I like to share some information about the SD card reading speed.
>>=20
>> While exchanging the old boot sequence with then new one I once =
booted
>> with the new boot sequence but without an /boot/loader.rc file.
>>=20
>> The reading speed achieved with
>> dd if=3D/dev/mmcsd0 of=3D/dev/null bs=3D1m count=3D100
>> from the SD card was 10 Mbyte/sec.
>>=20
>> After creating the file with "echo 'fdt addr 0x100' > =
/boot/loader.rc"
>> and rebooting the reading speed achieved from the SD card was down to
>> 2.8 Mbyte/sec.
>>=20
>> Although the SDHCI frequency was shown on the second boot as double =
high as
>> on the first (100MHz versus 50MHz) the reading speed was much lower.
>> I think the cause was the mmcsd bus bit width which was reported as 4 =
at
>> the first and as 1 at the second boot.
>=20
> At this time, it must be 50. if use 100MHz, some cards cant be used =
anymore.
> Howerver, probably 4bit transfer is OK.

33MHz is the top speed for SD cards.  There's an extension to make them =
go as fast as 50MHz.  Most SD cards cope at 50MHz without enabling the =
extension and even more when enabled.  The current common code tries to =
enable things properly, but relies on the host bridge adapter driver to =
set the clock properly...  There may be some cruft here left over from =
the early Atmel legacy where the datasheet gave somewhat aggressive =
advise..

100MHz at one bit is 100Mbps.  50MHz 4 bit is 200Mbps.

I gotta get a pi to play around with this :)

Warner=



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