SD card experts wanted

[Johannes Stezenbach]

Hi,

On Wed, Aug 29, 2012 at 11:17:03PM +0200, Juergen Beisert wrote:
> any SD/MMC card experts here? I have trouble with the current SD/MMC detection 
> code in Barebox.
> The function sd_change_freq() in the file 'drivers/mci/mci-core.c' tries to 
> detect the max. transfer frequency the attached card can handle. Nothing 
> special here, but the following line in this routine fails on my system:
> 
>   if ((__be32_to_cpu(switch_status[4]) & 0x0f000000) == 0x01000000)
> 
> It checks the clock speed capabilities returned from the card. As far as I 
> understand the "simplified SD card spec" the "0x01000000" means SDR25 speed, 

I think your analysis is correct.

> which means still "single transfer per clock at 25 MHz clock rate". But the 
> sd_change_freq() routine sets the MMC_MODE_HS bit for this case in the card's 
> capabilities variable. The remaining routines in 'drivers/mci/mci-core.c' 
> then use 50 MHz for the transfer speed.

IMHO mci_startup_sd() is buggy.

> This behaviour became an issue here, as my SDHC supports 50 MHz, but the 
> attached card only up to 25 MHz. After switching to 50 MHz, the card wasn't 
> able to respond anymore.
> For SDHCs which supports only up to 25 MHz this misinterpretation is no issue, 
> as the routines still use the 25 MHz, and the SD card can still respond.
> 
> "simplified SD card spec" talks about the bit's meaning: seems the regular 
> speed was SDR12 which means 12.5 MHz. Then SDR25 cards appear, which are also 
> called "high speed card". Could the MMC_MODE_HS just mean this "high speed" 
> and using it at 50 MHz is just some kind of typo? Or do MMCs act differently 
> and the code mixes SD and MMC features?
> 
> Any pointers would help.

MMC has the following speed modes:

- legacy 0-26MHz
- high speed SDR 0-52MHz
- high speed DDR 0-52MHz
- HS200 SDR 0-200MHz

while SD has:

- SDR12
- SDR25
- SDR50
- SDR104
- DDR50

So my guess is that MMC_MODE_HS maps to SDR25 for SD-only,
and MMC_MODE_HS_52MHz maps to SDR50 for SD and MMC.

(Apparently mmc_change_freq always sets MMC_MODE_HS, it is
meaningless for MMC.)

Thus, mci_startup_sd() should do:
	if (mci->card_caps & MMC_MODE_HS)
		mci_set_clock(mci, 25000000);
	else
		mci_set_clock(mci, 12500000);


Johannes