linux-kernel - Re: [PATCH v6 2/3] mmc: dw_mmc: Honor requests to set the clock to 0 (turn off clock)

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Message-id: <521B1A87.4000500@samsung.com>
Date:	Mon, 26 Aug 2013 18:06:15 +0900
From:	Jaehoon Chung <jh80.chung@...sung.com>
To:	Seungwon Jeon <tgih.jun@...sung.com>
Cc:	'Doug Anderson' <dianders@...omium.org>,
	'Chris Ball' <cjb@...top.org>,
	'Jaehoon Chung' <jh80.chung@...sung.com>,
	'James Hogan' <james.hogan@...tec.com>,
	'Grant Grundler' <grundler@...omium.org>,
	'Alim Akhtar' <alim.akhtar@...sung.com>,
	'Abhilash Kesavan' <a.kesavan@...sung.com>,
	'Tomasz Figa' <tomasz.figa@...il.com>,
	'Olof Johansson' <olof@...om.net>, linux-mmc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/3] mmc: dw_mmc: Honor requests to set the clock to 0
 (turn off clock)

On 08/26/2013 01:34 PM, Seungwon Jeon wrote:
> On Fri, August 23, 2013, Doug Anderson wrote:
>> Previously the dw_mmc driver would ignore any requests to disable the
>> card's clock.  This doesn't seem like a good thing in general, but had
>> one extra bad side effect in the following situtation:
>> * mmc core would set clk to 400kHz at boot time while initting
>> * mmc core would set clk to 0 since no card, but it would be ignored.
>> * suspend to ram and resume; clocks in the dw_mmc IP block are now 0
>>   but dw_mmc thinks that they're 400kHz (it ignored the set to 0).
>> * insert card
>> * mmc core would set clk to 400kHz which would be considered a no-op.
>>
>> Note that if there is no card in the slot and we do a suspend/resume
>> cycle, we _do_ still end up with differences in a dw_mmc register
>> dump, but the differences are clock related and we've got the clock
>> disabled both before and after, so this should be OK.
>>
>> Signed-off-by: Doug Anderson <dianders@...omium.org>
>> ---
>> Changes in v6:
>> - Replaces previous pathes that ensured saving/restoring clocks.
>>
>>  drivers/mmc/host/dw_mmc.c | 21 +++++++++++----------
>>  1 file changed, 11 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
>> index ee5f167..f6c7545 100644
>> --- a/drivers/mmc/host/dw_mmc.c
>> +++ b/drivers/mmc/host/dw_mmc.c
>> @@ -635,7 +635,11 @@ static void dw_mci_setup_bus(struct dw_mci_slot *slot, bool force_clkinit)
>>  	u32 div;
>>  	u32 clk_en_a;
>>
>> -	if (slot->clock != host->current_speed || force_clkinit) {
>> +	if (slot->clock == 0) {
>> +		mci_writel(host, CLKENA, 0);
>> +		mci_send_cmd(slot,
>> +			     SDMMC_CMD_UPD_CLK | SDMMC_CMD_PRV_DAT_WAIT, 0);
> Basically dw_mmc driver uses host's low power mode(auto clock gating)
> So, how about keeping origin code rather than programming clock setting to '0'?
Right, Dw-mmc controller can use the Low-power mode. 
This mode is functionality like clock-gating. Well, i didn't know what benefit we get, if set to 0.

Best Regards,
Jaehoon Chung
>  
>> +	} else if (slot->clock != host->current_speed || force_clkinit) {
> And then, if condition('slot->clock is not zero') is added in order to allow to set clock,
> print messages which Jaehoon pointed would be solved.
> 
> Thanks,
> Seungwon Jeon
> 
>>  		div = host->bus_hz / slot->clock;
>>  		if (host->bus_hz % slot->clock && host->bus_hz > slot->clock)
>>  			/*
>> @@ -675,9 +679,8 @@ static void dw_mci_setup_bus(struct dw_mci_slot *slot, bool force_clkinit)
>>  		/* inform CIU */
>>  		mci_send_cmd(slot,
>>  			     SDMMC_CMD_UPD_CLK | SDMMC_CMD_PRV_DAT_WAIT, 0);
>> -
>> -		host->current_speed = slot->clock;
>>  	}
>> +	host->current_speed = slot->clock;
>>
>>  	/* Set the current slot bus width */
>>  	mci_writel(host, CTYPE, (slot->ctype << slot->id));
>> @@ -807,13 +810,11 @@ static void dw_mci_set_ios(struct mmc_host *mmc, struct mmc_ios *ios)
>>
>>  	mci_writel(slot->host, UHS_REG, regs);
>>
>> -	if (ios->clock) {
>> -		/*
>> -		 * Use mirror of ios->clock to prevent race with mmc
>> -		 * core ios update when finding the minimum.
>> -		 */
>> -		slot->clock = ios->clock;
>> -	}
>> +	/*
>> +	 * Use mirror of ios->clock to prevent race with mmc
>> +	 * core ios update when finding the minimum.
>> +	 */
>> +	slot->clock = ios->clock;
>>
>>  	if (drv_data && drv_data->set_ios)
>>  		drv_data->set_ios(slot->host, ios);
>> --
>> 1.8.3
>>
>> --
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> 
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