linux-kernel - Re: [PATCH v2] mmc: sdhci-msm: Boost controller core clock

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Message-Id: <942B071D-D7F6-4D20-916C-BBE9096146E9@linaro.org>
Date:	Wed, 16 Dec 2015 13:44:01 +0200
From:	"Ivan T. Ivanov" <ivan.ivanov@...aro.org>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	Stephen Boyd <sboyd@...eaurora.org>,
	Bjorn Andersson <bjorn.andersson@...ymobile.com>,
	Georgi Djakov <georgi.djakov@...aro.org>,
	Bjorn Andersson <bjorn@...o.se>,
	Peter Griffin <peter.griffin@...aro.org>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH v2] mmc: sdhci-msm: Boost controller core clock


> On Dec 16, 2015, at 12:18 PM, Ulf Hansson <ulf.hansson@...aro.org> wrote:
> 
> [...]
> 
>>> It seems like a reasonable assumption that the controller can't cope
>>> with a higher clock rate than 100 MHz as "input" clock. That would
>>> then mean that there are different versions of the controller, as it
>>> seems like for some version it's fine with 200MHz and for some 100MHz.
>>> 
>>> According to the DT compatible strings, *one* version is currently
>>> supported, "qcom,sdhci-msm-v4"...
>> 
>> The same version of hardware is there 4 times. The difference is
>> the maximum clock frequency supported by them is different. In
>> downstream kernels we've handled this by trimming the frequency
>> tables for the different controllers in the clock driver.
>> Setting the clock to INT_MAX will make it run at 400MHz, which
>> doesn't look to be supported by anything besides sdc1 on 8974ac.
>> 
>>> 
>>> I see two viable solutions. One would be to limit the clock rate
>>> depending on the version of the controller (new compatible strings
>>> needs to be added). Another one would be to limit the clock rate by
>>> using the existing DT binding for max-frequency, and thus do a
>>> clk_set_rate(mmc->f_max) during probe.
>>> 
>> 
>> I'd rather see that done via OPP tables in DT, but I suppose
>> max-frequency is fine too. We'll need to use OPPs soon enough
>> because there's a voltage associated with that frequency.
>> 
>> In case you're wondering, the max frequency for sdc1 on 8974ac is
>> 400MHz. If it's just a plain 8974pro then the max frequency is
>> 200MHz. Otherwise, sdc2 maxes out at 200Mhz and sdc3 and sdc4 max
>> out at 100MHz.
>> 
> 
> I think we have reached a consensus on the viable options.
> 
> As we haven't heard from Ivan, do someone want to send a patch for
> this or shall we just revert $subject patch (which I can deal with)?

I am following this discussion, but I am not sure what I could do. 

If my understanding is correct, even if controllers report same version,
they don’t support 400MHz core clock. 

Initial patch fixes real issue. I am voting for “max-frequency” DT property.

I don’t have 8974ac, so I can not test the change.

Regards,
Ivan

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