> -----Original Message----- > From: Sascha Hauer [mailto:s.hauer@xxxxxxxxxxxxxx] > Sent: Monday, July 16, 2012 1:33 AM > To: George Pontis > Cc: barebox@xxxxxxxxxxxxxxxxxxx > 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)); _______________________________________________ barebox mailing list barebox@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/barebox
