Re: [PATCH 2/2] mmc: sh_mmcif: mmc->f_max should be half of the bus clock

[Simon Horman <horms@verge.net.au>]

On Tue, Mar 27, 2012 at 09:53:33AM +0200, Guennadi Liakhovetski wrote:
> On Mon, 26 Mar 2012, Guennadi Liakhovetski wrote:
> 
> > On Mon, 26 Mar 2012, Magnus Damm wrote:
> > 
> > > On Mon, Mar 26, 2012 at 2:52 PM, Simon Horman <horms@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.liakhovetski@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.
> 
> As you see in the email header, that last email was sent about 24 hours 
> ago, it just got stuck on my mail server due to some local problem... 
> After the recent discussions on this topic, this mail is outdated and 
> should be disregarded. I certainly agree to using clk / 512 
> unconditionally for all host frequencies, which also was my original 
> suggestion in
> 
> http://article.gmane.org/gmane.linux.kernel.mmc/13556

Indeed it was your suggestion and a good one too.