RE: [PATCH v1 1/1] mmc: core: fix power class selection

["Subhash Jadavani" <subhashj@codeaurora.org>]

Hi Porzio,

> -----Original Message-----
> From: Luca Porzio (lporzio) [mailto:lporzio@micron.com]
> Sent: Tuesday, April 03, 2012 10:36 PM
> To: Subhash Jadavani; 'Saugata Das'
> Cc: linux-mmc@vger.kernel.org; linux-arm-msm@vger.kernel.org;
> girish.shivananjappa@linaro.org; Danilo Caraccio (dcaracci)
> Subject: RE: [PATCH v1 1/1] mmc: core: fix power class selection
> 
> Hi,
> 
> >
> > Yes, this makes sense. With current implementation,
> > mmc_select_powerclass() should never really fail which means failure
> > should be treated as fatal and we should really skip the card
initialization.
> > This patch is already pushed to mmc-next. So I will post another patch
> > (by next week as I will be on vacation in this week) to skip the card
> > initialization if mmc_select_powerclass fails.
> >
> > Is this ok?
> >
> > Regards,
> > Subhash
> >
> 
> There was an error in the public JEDEC eMMC 4.5 version: PWR_CL_200_360 =
> EXT_CSD[237] is wrongly defined because HS200 can be supported only @1.2V
> or 1.8V. New Jedec spec revision might fix this.
> 
> I and Danilo did a quick check in the power class selection routines and
it looks
> like the power class is set according to the OCR parsing.
> The OCR parsing to check the supported voltage range does not totally
cover
> the device capabilities because Vccq = 1.2V support is not encoded in the
> register.

According to my understanding, "vvv" in "PWR_CL_ff_vvv" (or
"PWR_CL_DDR_ff_vvv") indicates is VDD voltage not the VCCQ voltage. So we
shouldn't bother about what is the vccq level to decide which particular
field to read for power class. Please correct me if this understanding is
wrong.

Regards,
Subhash

> 
> If the above is true, this implies that indexes EXT_CSD_PWR_CL_200_360
will
> lead to an error.
> 
> DEVICE_TYPE = EXT_CSD[196] is more useful because it provides information
> on the Speed Rate and Voltage Range supported by the device.
> 
> Cheers,
>     Luca