On Sun, Dec 21, 2014 at 7:01 PM, Tim Kryger <tim.kryger@xxxxxxxxx> wrote: > On Wed, Dec 17, 2014 at 11:57 AM, Bjorn Andersson <bjorn@xxxxxxx> wrote: > >> I'm somewhat puzzled to what benefit 52221610d brings after bringing >> back the write of BIT(0). Is it just that we don't hit the BUG() on >> non-standard voltages? > > It is to allow the use of external regulators that are capable of > supplying a standard SD/MMC voltage but none of the standard SDHCI > voltages. > >> The full paragraph regarding BIT(0) reads: >> >> Before setting this bit, the SD Host Driver shall set SD Bus Voltage >> Select. If the >> Host Controller detects the No Card state, this bit shall be cleared. >> If this bit is cleared, the Host Controller shall immediately stop >> driving CMD and >> DAT[3:0] (tri-state) and drive SDCLK to low level (Refer to Section 2.2.14). >> >> So the Qualcomm HW engineers implemented the last "shall", but if >> someone else (what did nvidia do here?) also implemented the first >> "shall"s then we're back at needing a full revert of 52221610d. > > It is difficult to predict how non-compliant host controllers will > behave in the area where they have chosen to deviate from the > standard. > > Do controllers that lack internal regulators claim to support 1.8, > 3.0, or 3.3v in the host capabilities register? Or will they set none > of these bits? > > They lack the ability to influence the externally supplied VDD but > will they place requirements on the values of the SD Bus Voltage > Select field? > > "If the Host Driver selects an unsupported voltage in the SD Bus > Voltage Select field, the Host Controller may ignore writes to SD Bus > Power and keep its value at zero." > > I would hope that controllers that fail to implement the regulator > would allow the SD Bus Voltage Select field to be set to any value. > > We have established that it is okay to leave the Voltage Select as > zero in the Broadcom, Qualcomm, and Samsung implementations. > > It would be nice to get confirmation that this is also the case for > other implementations that rely on an external regulator. I took a look at Nvidia's host controller in the Tegra124 SoC on a Jetson TK1 board. Fortunately, it behaves like the Qualcomm chip and only requires the LSB be set. -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
