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Date:   Wed, 26 Aug 2020 18:52:46 +0200
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Lukasz Stelmach <l.stelmach@...sung.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Kukjin Kim <kgene@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org, netdev@...r.kernel.org,
        devicetree@...r.kernel.org, m.szyprowski@...sung.com,
        b.zolnierkie@...sung.com
Subject: Re: [PATCH 1/3] net: ax88796c: ASIX AX88796C SPI Ethernet Adapter
 Driver

On Wed, Aug 26, 2020 at 06:45:33PM +0200, Krzysztof Kozlowski wrote:
> On Wed, Aug 26, 2020 at 04:59:09PM +0200, Lukasz Stelmach wrote:
 > >> +#include <linux/of.h>
> > >> +#endif
> > >> +#include <linux/crc32.h>
> > >> +#include <linux/etherdevice.h>
> > >> +#include <linux/ethtool.h>
> > >> +#include <linux/gpio/consumer.h>
> > >> +#include <linux/init.h>
> > >> +#include <linux/io.h>
> > >> +#include <linux/kmod.h>
> > >> +#include <linux/mii.h>
> > >> +#include <linux/module.h>
> > >> +#include <linux/netdevice.h>
> > >> +#include <linux/platform_device.h>
> > >> +#include <linux/sched.h>
> > >> +#include <linux/spi/spi.h>
> > >> +#include <linux/timer.h>
> > >> +#include <linux/uaccess.h>
> > >> +#include <linux/usb.h>
> > >> +#include <linux/version.h>
> > >> +#include <linux/workqueue.h>
> > >
> > > All of these should be removed except the headers used directly in this
> > > header.
> > >
> > 
> > This is "private" header file included in all ax88796c_*.c files and
> > these are headers required in them. It seems more conveninet to have
> > them all listed in one place. What is the reason to do otherwise?
> 
> Because:
> 1. The header is included in other files (more than one) so each other
> compilation unit will include all these headers, while not all of them
> need. This has a performance penalty during preprocessing.
> 
> 2. You will loose the track which headers are needed, which are not. We
> tend to keep it local, which means each compilation unit includes stuff
> it needs. This helps removing obsolete includes later.
> 
> 3. Otherwise you could make one header, including all headers of Linux,
> and then include this one header in each of C files. One to rule them
> all.

... and I got one more:

4. Drivers sometimes get reused, extended or they parts got reused. If
a header includes more stuff, it simply will pollute all other units
trying to reuse it... making the re-usage difficult. This is less likely
reason, I mean, quite imaginary for this particular driver.

I don't expect pieces of this driver to be reused... but who knows. Many
times in the past in the kernel there was a huge work rewriting headers
in many files, because something was including something else and we
wanted to decouple these things.  Therefore following the pattern -
include stuff you explicitly use - helps in every case.

Best regards,
Krzysztof

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