Message-ID: <20210514020900.GB540@aspeedtech.com>
Date:   Fri, 14 May 2021 10:09:01 +0800
From:   Steven Lee <steven_lee@...eedtech.com>
To:     Andrew Jeffery <andrew@...id.au>
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

The 05/13/2021 08:42, Andrew Jeffery wrote:
> 
> 
> On Mon, 10 May 2021, at 15:33, Steven Lee wrote:
> > The 05/07/2021 15:36, Andrew Jeffery wrote:
> > > 
> > > 
> > > 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?
> > > 
> > 
> > Yes, you are right, aspeed_sdc_probe() calls clk_prepare_enable(),
> > aspeed_g6_clk_enable() does reset eMMC.
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/mmc/host/sdhci-of-aspeed.c#n496
> > 
> > However, the clock of eMMC is enabled in my u-boot(2019.04).
> > So it is retruned in the condition of aspeed_g6_clk_is_enabled() below
> > and doesn't reset eMMC.
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/clk/clk-ast2600.c?h=v5.12#n285
> 
> Okay, so what's the issue that the patch addresses? Is there a bug? 
> Presumably if u-boot isn't making use of the eMMC the clock won't be 
> on, so we'll do the reset if the kernel wants to make use of the 
> device. If u-boot _is_ using the eMMC, u-boot will have done the 
> correct clock enable/reset sequence and so the controller should be 
> ready to go?
> 
> The only potential issue remaining is u-boot leaving the controller in 
> a configuration the kernel isn't expecting when handing over. If that's 
> the issue then we've forgotten to do some specific initialisation (i.e. 
> not just reset the entire thing) of the controller in the driver probe 
> path, right?
> 

If DMA engine is used before probing eMMC in kernel stage,
eMMC controller may have unexpected behavior when re-exectuing
identifying process.
Thus, we need to reset at the beginning of kernel since
kernel is a new stage. We should not assume some one do something
before.

> FWIW I haven't recently seen any poor behaviour from the controller or 
> driver. For us (IBM) it seems to be working well since we sorted out 
> the phase configuration.
> 

Yes, you are right, everything work well currently. But, kernel is a new
stage, we cannot assume eMMC controller is at initial state when
entering kernel stage.

> Andrew

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