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

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Date:   Mon, 15 May 2017 16:36:45 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Stanislaw Gruszka <sgruszka@...hat.com>
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 <linux-wireless@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rt2x00: improve calling conventions for register accessors

On Mon, May 15, 2017 at 4:28 PM, Stanislaw Gruszka <sgruszka@...hat.com> wrote:
> 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.

How about adding 'depends on !KASAN' in Kconfig instead?

       Arnd