linux-kernel - Re: [PATCH] mmc: Reduce fOD to 200 kHz if possible

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20100915140840.GB1677@void.printf.net>
Date:	Wed, 15 Sep 2010 15:08:40 +0100
From:	Chris Ball <cjb@...top.org>
To:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
Cc:	Ben Nizette <bn@...sdigital.com>, linux-mmc@...r.kernel.org,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Pierre Ossman <pierre@...man.eu>
Subject: Re: [PATCH] mmc: Reduce fOD to 200 kHz if possible

Hi Haavard,

On Wed, Sep 15, 2010 at 12:51:38PM +0200, Haavard Skinnemoen wrote:
> Thanks for the references. IMO Hein's patch is overkill. There is
> absolutely no reason why 200 kHz should be a problem on any setup, and
> I haven't found any indication in any discussions that it is.

Pierre was worried that we'd ping-pong between having an f_min too high
for some cards and too low for others, and be breaking a new set of
cards each time we changed the value.  He may have been being overly
pessimistic, but I don't think being cautious comes with any significant
downsides here.

There's also a (small) performance concern when f_min gets low, though
I agree that it wouldn't be a problem at 200 kHz.

> The reason why fOD was set to 400 kHz in the first place is that some
> controllers have a very low f_min so running the initialization at that
> frequency causes problems. Which makes sense because the SD standard
> clearly says that the clock can't be slower than 100 kHz.
> 
> But I have never seen any reasons why we absolutely _have_ to run the
> clock at the maximum frequency allowed by the spec. In fact, Sascha
> Hauer, who was the one who changed the minimum clock frequency to 400
> kHz, said he would be fine with any frequency between 50 kHz and 400
> kHz [1].

I agree that we aren't trying to run at the maximum possible frequency,
just trying to avoid having to choose a single perfect value without
enough information on what it is.

Thanks,

-- 
Chris Ball   <cjb@...top.org>   <http://printf.net/>
One Laptop Per Child
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/