linux-kernel - Re: [PATCH] PCI/ASPM: Use 64-bit arithmetic instead of 32-bit

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Message-ID: <20180227202621.GC127842@bhelgaas-glaptop.roam.corp.google.com>
Date:   Tue, 27 Feb 2018 14:26:22 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI/ASPM: Use 64-bit arithmetic instead of 32-bit

On Tue, Feb 13, 2018 at 01:05:50PM -0600, Gustavo A. R. Silva wrote:
> Hi Andy,
> 
> Quoting Andy Shevchenko <andy.shevchenko@...il.com>:
> 
> > On Tue, Feb 13, 2018 at 6:59 PM, Gustavo A. R. Silva
> > <garsilva@...eddedor.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.

Changing it to u32 sounds good to me.  I can't remember why I chose
u64 to begin with, but it doesn't look necessary.

Thanks for cleaning this up!