Messages in this thread | | | Date | Wed, 15 Sep 2010 15:08:40 +0100 | From | Chris Ball <> | Subject | Re: [PATCH] mmc: Reduce fOD to 200 kHz if possible |
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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@laptop.org> <http://printf.net/> One Laptop Per Child
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