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

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Message-ID: <20180228205546.GL127842@bhelgaas-glaptop.roam.corp.google.com>
Date:   Wed, 28 Feb 2018 14:55:46 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>
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 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>
    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
>