Re: [PATCH] PCI/ASPM: Change type of threshold_ns variable to u32

[Gustavo A. R. Silva]

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@xxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxx>
>      [bhelgaas: changelog]
>      Signed-off-by: Bjorn Helgaas <helgaas@xxxxxxxxxx>

Thanks, Bjorn.
--
Gustavo

>      Reviewed-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
>
> > ---
> >   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