linux-kernel - Re: [PATCH] mmc: sdhci: fix minimum clock rate for v3 controller

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Date:   Wed, 15 Jan 2020 09:16:54 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     linux-mmc@...r.kernel.org, Ulf Hansson <ulf.hansson@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mmc: sdhci: fix minimum clock rate for v3 controller

On 14/01/20 5:53 pm, Michał Mirosław wrote:
> On Tue, Jan 14, 2020 at 03:08:08PM +0200, Adrian Hunter wrote:
>> On 2/01/20 12:51 pm, Michał Mirosław wrote:
>>> For SDHCIv3+ with programmable clock mode, minimal clock frequency is
>>> still base clock / max(divider). Minimal programmable clock frequency is
>>> always greater than minimal divided clock frequency. Without this patch,
>>> SDHCI uses out-of-spec initial frequency when multiplier is big enough:
>>>
>>> mmc1: mmc_rescan_try_freq: trying to init card at 468750 Hz
>>> [for 480 MHz source clock divided by 1024]
>>
>> The maximum divisor in programmable clock mode is 1024.  So I do not
>> understand what is wrong.  Can you explain some more?
> 
> This part of code misses the fact, that you can choose (switch) between
> base clock mode and programmable clock mode. The code in
> sdhci_calc_clk() already does the choosing part. This is actually
> required on high programmable clock base to get conformant frequency for
> the card initialization phase.

Ok, so please add that explanation to the commit message, and add a comment
in the code too.

> 
> Best Regards,
> Michał Mirosław
> 
> [...]
>>> index 275102c0a1bf..0036ddf85674 100644
>>> --- a/drivers/mmc/host/sdhci.c
>>> +++ b/drivers/mmc/host/sdhci.c
>>> @@ -3902,11 +3902,9 @@ int sdhci_setup_host(struct sdhci_host *host)
>>>  	if (host->ops->get_min_clock)
>>>  		mmc->f_min = host->ops->get_min_clock(host);
>>>  	else if (host->version >= SDHCI_SPEC_300) {
>>> -		if (host->clk_mul) {
>>> -			mmc->f_min = (host->max_clk * host->clk_mul) / 1024;
>>> +		if (host->clk_mul)
>>>  			max_clk = host->max_clk * host->clk_mul;
>>> -		} else
>>> -			mmc->f_min = host->max_clk / SDHCI_MAX_DIV_SPEC_300;
>>> +		mmc->f_min = host->max_clk / SDHCI_MAX_DIV_SPEC_300;
>>>  	} else
>>>  		mmc->f_min = host->max_clk / SDHCI_MAX_DIV_SPEC_200;