linux-kernel - Re: [PATCH 2/3] usb: host: pci: introduce PCI vendor ID for Netlogic

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Date:   Thu, 15 Mar 2018 10:26:32 +0100
From:   Oliver Neukum <oneukum@...e.com>
To:     Richard Leitner <richard.leitner@...data.com>,
        Richard Leitner <dev@...l1n.net>, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-usb@...r.kernel.org
Cc:     bhelgaas@...gle.com, mathias.nyman@...el.com,
        gregkh@...uxfoundation.org
Subject: Re: [PATCH 2/3] usb: host: pci: introduce PCI vendor ID for Netlogic

Am Mittwoch, den 14.03.2018, 16:44 +0100 schrieb Richard Leitner:
> On 03/14/2018 04:27 PM, Oliver Neukum wrote:
> > Am Mittwoch, den 14.03.2018, 14:31 +0100 schrieb Richard Leitner:
> > > 
> > Well, but it does not. Removing a redundant definition is a clear
> > benefit. But you are not removing a definition. You are introducing
> > a preprocessor constant. Why?
> > What is its benefit?
> 
> AFAIK pci_ids.h collects PCI vendor and device IDs in one single 
> point. As the PCI vendor ID of Netlogic is used in multiple files
> IMHO it would be a good idea to add it to pci_ids.h and furthermore
> remove it from arch/mips/include/asm/netlogic/xlp-hal/iomap.h (where
> it's currently defined).
> 
> Or am I getting things wrong?

I think so, yes. We are giving names to constants as a form
of comment or to change them at multiple places at once and
consistently.

So

#define XYZ_NETDEV_RESET_RETRIES	2

makes clearly sense. So does

#define XYZ_MAGIC_VALUE1	0xab4e

because it tells you that you have a magic value.
But you will never redefine a PCI vendor ID. In fact you
must not. And if you have a comparison like

dev->vID == 0x1234

if you change this to

dev->vID == SOME_VENDOR_ID

what good does this to you? You already knew it was a vendor ID.
Now you can name it at a glance. So what? If you have a device
you will have to check whether you have some OEM version. You
will always go and check the raw number. And if you have a log
and need to check whether the check will be true, you will have
a number.
Using a constant there is nothing but trouble. Yet one more grep.

	Regards
		Oliver