Atmel AT91 SD/MMC clock too fast

[George Pontis]

> -----Original Message-----
> From: Sascha Hauer [mailto:s.hauer at pengutronix.de]
> Sent: Monday, July 16, 2012 1:33 AM
> To: George Pontis
> Cc: barebox at lists.infradead.org
> Subject: Re: Atmel AT91 SD/MMC clock too fast
> 
> Hi George,
> 
> On Mon, Jul 16, 2012 at 12:22:13AM -0700, George Pontis wrote:
> > Due to integer truncation, the calculated divisor for the SD/MMC clock
may
> > be too low, resulting in a clock that is too fast. For example, the
> > at91sam9g45 is typically run with a master clock at 133 MHz. There is no
> > divisor possible that will result in a 25 or 50 MHz clock as is
typically
> > used for a SD card. The existing code will set the clock at 33 MHz or 66
> MHz
> > when 25 or 50 MHz is requested. A Kingston class 10 8G SDHC was found to
> not
> > work properly at 33 MHz, but did after patching. The patch, in this
case,
> > generated a 22.1 MHz clock ( 133/6 ). This is the same frequency that
this
> > card sees after booting Linux. If a perfect integer division is
possible,
> > the patch will provide an exact clock rate. If not, the frequency will
be
> > the closest possible without exceeding the target.
> >
> > The following was based on barebox 2012-07:
> >
> > --- a/drivers/mci/atmel_mci.c   2012-07-02 01:31:52.000000000 -0700
> > +++ b/drivers/mci/atmel_mci.c   2012-07-15 23:14:02.203872596 -0700
> > @@ -76,7 +76,8 @@
> >         unsigned int clk_in = clk_get_rate(host->clk);
> >
> >         if (clk_ios > 0) {
> > -               divider = (clk_in / clk_ios) / 2;
> > +               /* don't allow integer truncation to result in too small
a
> > divider */
> > +               divider = (clk_in + 2 * clk_ios - 1) / (2 * clk_ios);
> 
> DIV_ROUND_UP(clk_in, clk_ios / 2) instead?
> 
> Sascha
> 

Looks good, but it needs a divisor of 2*clk_ios. Using that macro:


--- a/drivers/mci/atmel_mci.c   2012-07-02 01:31:52.000000000 -0700
+++ b/drivers/mci/atmel_mci.c   2012-07-15 23:14:02.203872596 -0700
@@ -76,7 +76,8 @@
        unsigned int clk_in = clk_get_rate(host->clk);

        if (clk_ios > 0) {
-               divider = (clk_in / clk_ios) / 2;
+               /* don't allow integer truncation to result in too small a
divider */
+               divider = DIV_ROUND_UP( clk_in, (2 * clk_ios));