linux-kernel - Re: [PATCH v3 5/5] mmc: sdhci-of-aspeed: Assert/Deassert reset signal before probing eMMC

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Message-Id: <2a339218-19d7-4eea-a734-8053dd553dbb@www.fastmail.com>
Date:   Fri, 07 May 2021 17:06:19 +0930
From:   "Andrew Jeffery" <andrew@...id.au>
To:     "Steven Lee" <steven_lee@...eedtech.com>
Cc:     "Philipp Zabel" <p.zabel@...gutronix.de>,
        "Ulf Hansson" <ulf.hansson@...aro.org>,
        "Rob Herring" <robh+dt@...nel.org>,
        "Joel Stanley" <joel@....id.au>,
        "Adrian Hunter" <adrian.hunter@...el.com>,
        "Ryan Chen" <ryanchen.aspeed@...il.com>,
        "moderated list:ASPEED SD/MMC DRIVER" <linux-aspeed@...ts.ozlabs.org>,
        "moderated list:ASPEED SD/MMC DRIVER" <openbmc@...ts.ozlabs.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "moderated list:ARM/ASPEED MACHINE SUPPORT" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list" <linux-kernel@...r.kernel.org>,
        "Hongwei Zhang" <Hongweiz@....com>,
        "Ryan Chen" <ryan_chen@...eedtech.com>,
        "Chin-Ting Kuo" <chin-ting_kuo@...eedtech.com>
Subject: Re: [PATCH v3 5/5] mmc: sdhci-of-aspeed: Assert/Deassert reset signal before probing eMMC



On Fri, 7 May 2021, at 15:54, Steven Lee wrote:
> The 05/07/2021 09:32, Andrew Jeffery wrote:
> > 
> > 
> > On Thu, 6 May 2021, at 19:54, Philipp Zabel wrote:
> > > Hi Steven,
> > > 
> > > On Thu, May 06, 2021 at 06:03:12PM +0800, Steven Lee wrote:
> > > > +	if (info) {
> > > > +		if (info->flag & PROBE_AFTER_ASSET_DEASSERT) {
> > > > +			sdc->rst = devm_reset_control_get(&pdev->dev, NULL);
> > > 
> > > Please use devm_reset_control_get_exclusive() or
> > > devm_reset_control_get_optional_exclusive().
> > > 
> > > > +			if (!IS_ERR(sdc->rst)) {
> > > 
> > > Please just return errors here instead of ignoring them.
> > > The reset_control_get_optional variants return NULL in case the
> > > device node doesn't contain a resets phandle, in case you really
> > > consider this reset to be optional even though the flag is set?
> > 
> > It feels like we should get rid of the flag and leave it to the 
> > devicetree.
> > 
> 
> Do you mean adding a flag, for instance, "mmc-reset" in the
> device tree and call of_property_read_bool() in aspeed_sdc_probe()?
> 
> > I'm still kind of surprised it's not something we want to do for the 
> > 2400 and 2500 as well.
> > 
> 
> Per discussion with the chip designer, AST2400 and AST2500 doesn't need
> this implementation since the chip design is different to AST2600.

So digging a bit more deeply on this, it looks like the reset is 
already taken care of by drivers/clk/clk-ast2600.c in the 
clk_prepare_enable() path.

clk-ast2600 handles resets when enabling the clock for most peripherals:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/clk/clk-ast2600.c?h=v5.12#n276

and this is true for both the SD controller and the eMMC controller:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/clk/clk-ast2600.c?h=v5.12#n94
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/clk/clk-ast2600.c?h=v5.12#n88

If this weren't the case you'd specify a reset property in the SD/eMMC 
devicetree nodes for the 2600 and then use 
devm_reset_control_get_optional_exclusive() as Philipp suggested. See 
the reset binding here:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/reset/reset.txt?h=v5.12

So on the surface it seems the reset handling in this patch is 
unnecessary. Have you observed an issue with the SoC that means it's 
required?

Andrew