linux-kernel - Re: [PATCH] PCI/ASPM: Change type of threshold

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Message-ID: <2a893793-b7a0-a13f-4b88-8cf28554d816@embeddedor.com>
Date:   Wed, 28 Feb 2018 21:14:55 -0600
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI/ASPM: Change type of threshold_ns variable to u32



On 02/28/2018 02:55 PM, Bjorn Helgaas wrote:
> On Tue, Feb 27, 2018 at 05:19:52PM -0600, Gustavo A. R. Silva wrote:
>> It seems that the expression threshold_us * 1000 will never exceed the
>> 32-bit limits [1]. So changing the type of threshold_ns from u64 to u32
>> seems sensible [2].
>>
>> [1] https://marc.info/?l=linux-kernel&m=151855021100725&w=2
>> [2] https://marc.info/?l=linux-kernel&m=151976318924615&w=2
>>
>> Addresses-Coverity-ID: 1462501
>> Signed-off-by: Gustavo A. R. Silva <garsilva@...eddedor.com>
> 
> Applied with Andy's Reviewed-by to pci/aspm for v4.17, thanks!
> 
> I included a more detailed analysis in the changelog:
> 
>      PCI/ASPM: Declare threshold_ns as u32, not u64
>      
>      aspm_calc_l1ss_info() computes l1_2_threshold in microseconds as:
>      
>        l1_2_threshold = 2 + 4 + t_common_mode + t_power_on;
>      
>      where t_common_mode is at most 255us:
>      
>        PCI_L1SS_CAP_CM_RESTORE_TIME  0x0000ff00   <-- 8 bits; <256us
>      
>      and t_power_on is at most 31 * 100us = 3100us:
>      
>        PCI_L1SS_CAP_P_PWR_ON_VALUE   0x00f80000   <-- 5 bits; <32
>        PCI_L1SS_CAP_P_PWR_ON_SCALE   0x00030000   <-- *2us, *10us, or *100us
>      
>      So l1_2_threshold is at most 2 + 4 + 255 + 3100 = 3361, which means
>      threshold_ns is at most 3361 * 1000 = 3361000, which easily fits in a
>      u32.
>      
>      Declare threshold_ns as u32, not u64.  This fixes a Coverity warning.
>      
>      Addresses-Coverity-ID: 1462501
>      Signed-off-by: Gustavo A. R. Silva <garsilva@...eddedor.com>
>      [bhelgaas: changelog]
>      Signed-off-by: Bjorn Helgaas <helgaas@...nel.org>

Thanks, Bjorn.
--
Gustavo

>      Reviewed-by: Andy Shevchenko <andy.shevchenko@...il.com>
> 
>> ---
>>   drivers/pci/pcie/aspm.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
>> index 57feef2..8633fc4 100644
>> --- a/drivers/pci/pcie/aspm.c
>> +++ b/drivers/pci/pcie/aspm.c
>> @@ -322,7 +322,7 @@ static u32 calc_l1ss_pwron(struct pci_dev *pdev, u32 scale, u32 val)
>>   
>>   static void encode_l12_threshold(u32 threshold_us, u32 *scale, u32 *value)
>>   {
>> -	u64 threshold_ns = threshold_us * 1000;
>> +	u32 threshold_ns = threshold_us * 1000;
>>   
>>   	/* See PCIe r3.1, sec 7.33.3 and sec 6.18 */
>>   	if (threshold_ns < 32) {
>> -- 
>> 2.7.4
>>