PCI/ASPM: Use 64-bit arithmetic instead of 32-bit

Message ID 20180213165948.GA14035@embeddedor.com
State New
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Series
  • PCI/ASPM: Use 64-bit arithmetic instead of 32-bit
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Commit Message

Gustavo A. R. Silva Feb. 13, 2018, 4:59 p.m.
Add suffix ULL to constant 1000 in order to give the compiler complete
information about the proper arithmetic to use. Notice that this
constant is used in a context that expects an expression of type
u64 (64 bits, unsigned).

The expression threshold_us * 1000 is currently being evaluated
using 32-bit arithmetic.

Addresses-Coverity-ID: 1462501
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
---
 drivers/pci/pcie/aspm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Andy Shevchenko Feb. 13, 2018, 6:29 p.m. | #1
On Tue, Feb 13, 2018 at 6:59 PM, Gustavo A. R. Silva
<garsilva@embeddedor.com> wrote:
> Add suffix ULL to constant 1000 in order to give the compiler complete
> information about the proper arithmetic to use. Notice that this
> constant is used in a context that expects an expression of type
> u64 (64 bits, unsigned).
>
> The expression threshold_us * 1000 is currently being evaluated
> using 32-bit arithmetic.

> -       u64 threshold_ns = threshold_us * 1000;
> +       u64 threshold_ns = threshold_us * 1000ULL;

Shouldn't be other way around, i.e.

(u64)threshold_us ?

But still the question. have you checked all callers? Does it even makes sense?
Gustavo A. R. Silva Feb. 13, 2018, 7:05 p.m. | #2
Hi Andy,

Quoting Andy Shevchenko <andy.shevchenko@gmail.com>:

> On Tue, Feb 13, 2018 at 6:59 PM, Gustavo A. R. Silva
> <garsilva@embeddedor.com> wrote:
>> Add suffix ULL to constant 1000 in order to give the compiler complete
>> information about the proper arithmetic to use. Notice that this
>> constant is used in a context that expects an expression of type
>> u64 (64 bits, unsigned).
>>
>> The expression threshold_us * 1000 is currently being evaluated
>> using 32-bit arithmetic.
>
>> -       u64 threshold_ns = threshold_us * 1000;
>> +       u64 threshold_ns = threshold_us * 1000ULL;
>
> Shouldn't be other way around, i.e.
>
> (u64)threshold_us ?
>

Either way works. The thing is that casting threshold_us to u64 may  
imply that there is something wrong with threshold_us, which does not  
seem to be the case. So adding the suffix ULL to the constant 1000 is  
good enough to make the expression be evaluated using 64-bit  
arithmetic instead of 32-bit.

But, again, either way works.

> But still the question. have you checked all callers? Does it even  
> makes sense?
>

The proposed patch was due to fact that currently threshold_ns is of  
type u64. But based on the following piece of code (which is the only  
piece of code from where encode_l12_threshold is being called):

          * Based on PCIe r3.1, sec 5.5.3.3.1, Figures 5-16 and 5-17, and
          * Table 5-11.  T(POWER_OFF) is at most 2us and T(L1.2) is at
          * least 4us.
          */
         l1_2_threshold = 2 + 4 + t_common_mode + t_power_on;
         encode_l12_threshold(l1_2_threshold, &scale, &value);

It seems to me that it makes no sense for threshold_ns to be of type  
u64, because the expression threshold_us * 1000 will never exceed the  
32-bit limits. So if you agree I can send a patch to change its type  
to u32 instead.

Thanks
--
Gustavo
Andy Shevchenko Feb. 13, 2018, 7:26 p.m. | #3
On Tue, Feb 13, 2018 at 9:05 PM, Gustavo A. R. Silva
<garsilva@embeddedor.com> wrote:

> It seems to me that it makes no sense for threshold_ns to be of type u64,
> because the expression threshold_us * 1000 will never exceed the 32-bit
> limits. So if you agree I can send a patch to change its type to u32
> instead.

Whatever Bjorn prefers.

Patch

diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
index 57feef2..684d326 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;
+	u64 threshold_ns = threshold_us * 1000ULL;
 
 	/* See PCIe r3.1, sec 7.33.3 and sec 6.18 */
 	if (threshold_ns < 32) {