On Mon, 26 Mar 2012, Magnus Damm wrote:
> On Mon, Mar 26, 2012 at 2:52 PM, Simon Horman <ho...@verge.net.au> wrote:
> > On Mon, Mar 26, 2012 at 02:45:30PM +0900, Yusuke Goda wrote:
> >> Hi Simon-san, Guennadi-san
> >>
> >> (2012/03/26 7:30), Simon Horman wrote:
> >> > On Sat, Mar 24, 2012 at 07:06:31PM +0100, Guennadi Liakhovetski wrote:
> >> >> On Wed, 21 Mar 2012, Simon Horman wrote:
> >> >>
> >> >>> mmc->f_max should be half of the bus clock.
> >> >>> And now that mmc->f_max is not equal to the bus clock the
> >> >>> latter should be used directly to calculate mmc->f_min.
> >> >>
> >> >> The patch seems correct as it stands, however, looking at it - does
> >> >> anyone
> >> >> understands why that "close to 400kHz" comment and such a complicated
> >> >> calculation? Shouldn't it be just host->clk / 512 always? Maybe this
> >> >> should be a separate patch, so, for this one
> >> >>
> >> >> Acked-by: Guennadi Liakhovetski <g.liakhovet...@gmx.de>
> >> >
> >> > Hi Guennadi,
> >> >
> >> > that code seems to date back to the original driver submission
> >> > made by Goda-san. I have CCed him as perhaps he recalls why
> >> > the code is like it is.
> >> I thought to get closer to 400kHz if possible.
> >> Probably even host->clk / 512 does not have any problem.
> >
> > Sorry for my ignorance, is ~400kHz desirable for some reason?
>
> The 400kHz frequency is used during initialization of the SD card.
> Simply put, the SD frequency starts low out low and is then changed to
> something higher depending on the capability of the memory card and
> the host controller. Please have a look at the simplified SD
> specification for more details.
I see, so, we want something like
shift = fls(host->clk / 400000 - 1);
mmc->f_min = host->clk >> shift;
eventually with some rounding, depending on our preferences.
Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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