linux-kernel - Re: [PATCH v2 4/7] crypto: sun4i-ss: handle BigEndian for cipher

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Date:   Wed, 23 Sep 2020 20:59:02 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     LABBE Corentin <clabbe@...libre.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Maxime Ripard <mripard@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>, "# 3.4.x" <stable@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 4/7] crypto: sun4i-ss: handle BigEndian for cipher

On Wed, Sep 23, 2020 at 8:08 PM LABBE Corentin <clabbe@...libre.com> wrote:
> On Wed, Sep 23, 2020 at 04:00:32PM +0200, Arnd Bergmann wrote:
> > On Sun, Sep 20, 2020 at 8:37 PM Corentin Labbe <clabbe@...libre.com> wrote:
> > > diff --git a/drivers/crypto/allwinner/sun4i-ss/sun4i-ss-cipher.c b/drivers/crypto/allwinner/sun4i-ss/sun4i-ss-cipher.c
> > > index c6c25204780d..a05889745097 100644
> > > --- a/drivers/crypto/allwinner/sun4i-ss/sun4i-ss-cipher.c
> > > +++ b/drivers/crypto/allwinner/sun4i-ss/sun4i-ss-cipher.c
> > > @@ -52,13 +52,13 @@ static int noinline_for_stack sun4i_ss_opti_poll(struct skcipher_request *areq)
> > >
> > >         spin_lock_irqsave(&ss->slock, flags);
> > >
> > > -       for (i = 0; i < op->keylen; i += 4)
> > > -               writel(*(op->key + i / 4), ss->base + SS_KEY0 + i);
> > > +       for (i = 0; i < op->keylen / 4; i++)
> > > +               writel(cpu_to_le32(op->key[i]), ss->base + SS_KEY0 + i * 4);
> >
> > I suspect what you actually want here is writesl() in place of the
> > loop. This skips the byteswap on big-endian, rather than swapping
> > each word twice.
> >
> > The point is that this register seems to act as a FIFO for a byte-stream
> > rather than a 32-bit fixed-endian register.
>
> Thanks, using writesl() fixes the warning, but I need to keep the loop
> since the register is different each time.

Ah, I see. I thought we had an interface for that as well, but I can't
find it now. I see memcpy_toio32() in one driver, but that implementation
appears to be wrong here (and probably also wrong for the machine
it was meant for)

There is the regular memcpy_toio(), but on big-endian Arm that
turns into a per-byte copy, which might either not work on your
hardware or be too slow.

There is also __iowrite32_copy(), which is not what I had remembered
but does seem to do what you want here.

> Or does it is better to use directly __raw_writel() ?

__raw_writel() is not very portable, so I would avoid that in normal
device drivers even when you only run them on specific hardware.

      Arnd