linux-kernel - Re: [PATCH] mmc: dw_mmc: remove UBSAN warning in dw_mci_setup

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Date:	Fri, 17 Jun 2016 13:07:25 +0900
From:	Seung-Woo Kim <sw0312.kim@...sung.com>
To:	Jaehoon Chung <jh80.chung@...sung.com>
Cc:	ulf.hansson@...aro.org, linux-mmc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Seung-Woo Kim <sw0312.kim@...sung.com>
Subject: Re: [PATCH] mmc: dw_mmc: remove UBSAN warning in dw_mci_setup_bus()

Hello Jaehoon,

On 2016년 06월 17일 10:30, Jaehoon Chung wrote:
> Hi Seung-Woo,
> 
> On 06/10/2016 10:29 AM, Seung-Woo Kim wrote:
>> Hi Jaehoon,
>>
>> On 2016년 06월 09일 21:38, Jaehoon Chung wrote:
>>> Hi Seung-Woo,
>>>
>>> On 06/08/2016 01:07 PM, Seung-Woo Kim wrote:
>>>> This patch removes following UBSAN warnings in dw_mci_setup_bus().
>>>> The warnings are caused because of shift with more than 31 on 32
>>>> bit variable, so this patch fixes to shift only for less than 32.
>>>>

<snip.>

>>>>
>>>> diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
>>>> index 2cc6123..dff045e 100644
>>>> --- a/drivers/mmc/host/dw_mmc.c
>>>> +++ b/drivers/mmc/host/dw_mmc.c
>>>> @@ -1099,7 +1099,8 @@ static void dw_mci_setup_bus(struct dw_mci_slot *slot, bool force_clkinit)
>>>>  
>>>>  		div = (host->bus_hz != clock) ? DIV_ROUND_UP(div, 2) : 0;
>>>>  
>>>> -		if ((clock << div) != slot->__clk_old || force_clkinit)
>>>> +		if (((div < 32) ? (clock << div) : 0) != slot->__clk_old ||
>>>
>>> Well, we don't expect that clock is 0.
>>> if clock is 0, it should be passed to " if (!clock)"..
>>>
>>> During initializing card, clock is 400KHz or less..I understood what you want to fix.
>>> But I taught this is not correct.
>>
>> It seems slot->__clk_old is not really clock but a kind of value to
>> store both clock and div in one variable. And at least, my compiler
>> calculates right shift with more than 32 as 0. I am not sure there is
>> other proper value for the case.
>>
>> By the way, in my test environment, clock is calculated as like
>> following steps:
>> mmc_host mmc0: Bus speed (slot 0) = 200000000Hz (slot req 400000Hz,
>> actual 400000HZ div = 250)
>> mmc_host mmc0: Bus speed (slot 0) = 200000000Hz (slot req 200000000Hz,
>> actual 200000000HZ div = 0)
>> mmc_host mmc0: Bus speed (slot 0) = 200000000Hz (slot req 52000000Hz,
>> actual 50000000HZ div = 2)
>> mmc_host mmc0: Bus speed (slot 0) = 400000000Hz (slot req 52000000Hz,
>> actual 50000000HZ div = 4)
>> mmc_host mmc0: Bus speed (slot 0) = 400000000Hz (slot req 200000000Hz,
>> actual 200000000HZ div = 1)
>>
>> and only the first case is reported as the warning. If you let me know
>> any proper value, then I will fix with the value.
> 
> I think that "clock << div" and__old_clock are used for checking whether clock is changed.
> So it doesn't need to rotate with div.
> 
> Could you check the below codes?
> 
> if ((clock != slot->clk_old) || force_clkinit)
> ...
> 
> slot->__clk_old = clock;

>From the comment about alignment of slot->__clk_old, it says that "keep
the clock with reflecting clock dividor". So to me, it is better also
adding slot->__dlv with your suggestion. I will send patch like it, soon.

Regards,
- Seung-Woo Kim

> 
> Best Regards,
> Jaehoon Chung
> 
>>
>> Thanks,
>> - Seung-Woo Kim
>>
>>
>>>
>>>
>>>> +		    force_clkinit)
>>>>  			dev_info(&slot->mmc->class_dev,
>>>>  				 "Bus speed (slot %d) = %dHz (slot req %dHz, actual %dHZ div = %d)\n",
>>>>  				 slot->id, host->bus_hz, clock,
>>>> @@ -1129,7 +1130,7 @@ static void dw_mci_setup_bus(struct dw_mci_slot *slot, bool force_clkinit)
>>>>  		mci_send_cmd(slot, sdmmc_cmd_bits, 0);
>>>>  
>>>>  		/* keep the clock with reflecting clock dividor */
>>>> -		slot->__clk_old = clock << div;
>>>> +		slot->__clk_old = (div < 32) ? (clock << div) : 0;

<snip.>

-- 
Seung-Woo Kim
Samsung Software R&D Center
--