linux-kernel - Re: [PATCH v5 03/14] PCI: Add pcie_bandwidth_capable() to compute max supported link bandwidth

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Message-ID: <20180403140552.GB60020@bhelgaas-glaptop.roam.corp.google.com>
Date:   Tue, 3 Apr 2018 09:05:52 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jacob Keller <jacob.keller@...il.com>
Cc:     Tal Gilboa <talgi@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Jacob Keller <jacob.e.keller@...el.com>,
        Ariel Elior <ariel.elior@...ium.com>,
        Ganesh Goudar <ganeshgr@...lsio.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        everest-linux-l2@...ium.com, intel-wired-lan@...ts.osuosl.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH v5 03/14] PCI: Add pcie_bandwidth_capable() to compute
 max supported link bandwidth

On Mon, Apr 02, 2018 at 05:30:54PM -0700, Jacob Keller wrote:
> On Mon, Apr 2, 2018 at 7:05 AM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > +/* PCIe speed to Mb/s reduced by encoding overhead */
> > +#define PCIE_SPEED2MBS_ENC(speed) \
> > +       ((speed) == PCIE_SPEED_16_0GT ? (16000*(128/130)) : \
> > +        (speed) == PCIE_SPEED_8_0GT  ?  (8000*(128/130)) : \
> > +        (speed) == PCIE_SPEED_5_0GT  ?  (5000*(8/10)) : \
> > +        (speed) == PCIE_SPEED_2_5GT  ?  (2500*(8/10)) : \
> > +        0)
> > +
> 
> Should this be "(speed * x ) / y" instead? wouldn't they calculate
> 128/130 and truncate that to zero before multiplying by the speed? Or
> are compilers smart enough to do this the other way to avoid the
> losses?

Yep, thanks for saving me yet more embarrassment.