linux-kernel - Re: [PATCH] rt2x00: improve calling conventions for register accessors

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Message-ID: <20170515142520.GA13996@redhat.com>
Date:   Mon, 15 May 2017 16:28:01 +0200
From:   Stanislaw Gruszka <sgruszka@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Helmut Schaa <helmut.schaa@...glemail.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Daniel Golle <daniel@...rotopia.org>,
        Mathias Kresin <dev@...sin.me>,
        Johannes Berg <johannes.berg@...el.com>,
        Tomislav Požega <pozega.tomislav@...il.com>,
        Serge Vasilugin <vasilugin@...dex.ru>,
        Roman Yeryomin <roman@...em.lv>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rt2x00: improve calling conventions for register
 accessors

On Mon, May 15, 2017 at 03:46:55PM +0200, Arnd Bergmann wrote:
> With CONFIG_KASAN enabled and gcc-7, we get a warning about rather high
> stack usage (with a private patch set I have to turn on this warning,
> which I intend to get into the next kernel release):
> 
> wireless/ralink/rt2x00/rt2800lib.c: In function 'rt2800_bw_filter_calibration':
> wireless/ralink/rt2x00/rt2800lib.c:7990:1: error: the frame size of 2144 bytes is larger than 1536 bytes [-Werror=frame-larger-than=]
> 
> The problem is that KASAN inserts a redzone around each local variable that
> gets passed by reference, and the newly added function has a lot of them.
> We can easily avoid that here by changing the calling convention to have
> the output as the return value of the function. This should also results in
> smaller object code, saving around 4KB in .text with KASAN, or 2KB without
> KASAN.
> 
> Fixes: 41977e86c984 ("rt2x00: add support for MT7620")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/net/wireless/ralink/rt2x00/rt2800lib.c | 319 +++++++++++++------------
>  1 file changed, 164 insertions(+), 155 deletions(-)

We have read(, &val) calling convention since forever in rt2x00 and that
was never a problem. I dislike to change that now to make some tools
happy, I think problem should be fixed in the tools instead.

Stanislaw