linux-kernel - Re: [PATCH v2] mmc: core: add mmc-card hardware reset enable support

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Date:   Tue, 11 Apr 2017 12:43:39 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Richard Leitner <richard.leitner@...data.com>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Luca Porzio <lporzio@...ron.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Jaehoon Chung <jh80.chung@...sung.com>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Richard Leitner <dev@...l1n.net>
Subject: Re: [PATCH v2] mmc: core: add mmc-card hardware reset enable support

On 11 April 2017 at 10:17, Linus Walleij <linus.walleij@...aro.org> wrote:
> On Tue, Apr 11, 2017 at 9:31 AM, Richard Leitner
> <richard.leitner@...data.com> wrote:
>
>> Some eMMCs disable their hardware reset line (RST_N) by default. To enable
>> it the host must set the corresponding bit in ECSD. An example for such
>> a device is the Micron MTFCxGACAANA-4M.
>>
>> This patch adds a new mmc-card devicetree property to let the host enable
>> this feature during card initialization.
>>
>> Signed-off-by: Richard Leitner <richard.leitner@...data.com>
>
> Do we know *WHY* these cards disable their hardware reset lines?

Allow me to make a guess. In case the reset isn't enabled, the
internal eMMC card firmware don't monitor the pin for the reset. I
guess that could makes sense if SoC vendors has failed to properly
connect the pin, avoiding the eMMC card to be reset when it shouldn't.

>
> If it is just some random over-cautious panic thing we might consider
> just force re-enableing it, maybe with a warning in the dmesg, so we can
> always reset the card. No DT property needed.

There is actually already a DT property "cap-mmc-hw-reset"
(MMC_CAP_HW_RESET), which tells whether the eMMC reset is supported by
the host.

Perhaps we can consider to force-enabling it for the eMMC card, when
this property is set for the mmc host!? At least, inventing yet
another binding doesn't make sense to me.

>
> Putting some people who work for eMMC vendors in the To: line so they
> can say if they know about this.

Yes, let's see what they say about it.

>
> Yours,
> Linus Walleij

Kind regards
Uffe