linux-kernel - Re: Possible regression with commit 52221610d

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Message-ID: <CAD7vxxJsrq2DQ=pDNn3yv=dMPK7vE=aoW9m36hBRdGypL2_i7g@mail.gmail.com>
Date:	Tue, 13 Jan 2015 21:00:01 -0800
From:	Tim Kryger <tim.kryger@...il.com>
To:	Bjorn Andersson <bjorn@...o.se>
Cc:	Ulf Hansson <ulf.hansson@...aro.org>,
	Alexandre Courbot <acourbot@...dia.com>,
	Sachin Kamat <spk.linux@...il.com>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alexandre Courbot <gnurou@...il.com>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: Possible regression with commit 52221610d

On Sun, Dec 21, 2014 at 7:01 PM, Tim Kryger <tim.kryger@...il.com> wrote:
> On Wed, Dec 17, 2014 at 11:57 AM, Bjorn Andersson <bjorn@...o.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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