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    SubjectRe: Possible regression with commit 52221610d
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    On Sun, Dec 21, 2014 at 7:01 PM, Tim Kryger <tim.kryger@gmail.com> wrote:
    > On Wed, Dec 17, 2014 at 11:57 AM, Bjorn Andersson <bjorn@kryo.se> 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.


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