linux-kernel - Re: [PATCH RESEND] PCI: dwc: put struct dw_pcie::{ep,pp} into a union to reduce its size

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Message-ID: <YFqWftATEbuxsJbn@rocinante>
Date:   Wed, 24 Mar 2021 02:31:42 +0100
From:   Krzysztof Wilczyński <kw@...ux.com>
To:     Alexander Lobakin <alobakin@...me>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Rob Herring <robh@...nel.org>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] PCI: dwc: put struct dw_pcie::{ep,pp} into a
 union to reduce its size

Hi Alexander,

Thank you for sending the patch over!

> A single dw_pcie entity can't be a root complex and an endpoint at
> the same time.

Nice catch!

A small nitpick: this would be Root Complex and Endpoint, as it's
customary to capitalise these.

Also, if you could capitalise the subject line - it could also perhaps
be simplified to something like, for example:

  Optimize struct dw_pcie to reduce its size

Feel free to ignore both suggestions, as these are just nitpicks.

> We can use this to reduce the size of dw_pcie by 80, from 280 to 200
> bytes (on x32, guess more on x64), by putting the related embedded
> structures (struct pcie_port and struct dw_pcie_ep) into a union.

[...]
> -	struct pcie_port	pp;
> -	struct dw_pcie_ep	ep;
> +	union {
> +		struct pcie_port	pp;
> +		struct dw_pcie_ep	ep;
> +	};
[...]

How did you measure the difference?  Often, people include pahole output
for the "before" and "after", so to speak, to showcase the difference
and/or improvement.  Do you have something like that handy?

Krzysztof