From: Maxime Ripard <maxime.ripard@free-electrons.com>
To: Oliver Schinagl <oliver+list@schinagl.nl>
Cc: linux-arm-kernel@lists.infradead.org, gregkh@linuxfoundation.org,
linux-kernel@vger.kernel.org, arnd@arndb.de
Subject: Re: [PATCH 1/2] Initial support for Allwinner's Security ID fuses
Date: Sat, 25 May 2013 14:22:08 +0200 [thread overview]
Message-ID: <20130525122208.GD17847@lukather> (raw)
In-Reply-To: <519FE0AE.2010102@schinagl.nl>
Hi Oliver,
On Fri, May 24, 2013 at 11:50:38PM +0200, Oliver Schinagl wrote:
> On 05/18/13 19:19, Oliver Schinagl wrote:
> <snip>
> >>>+
> >>>+
> >>>+/* We read the entire key, using a look up table. Returned is only the
> >>>+ * requested byte. This is of course slower then it could be and uses 4 times
> >>>+ * more reads as needed but keeps code a little simpler.
> >>>+ */
> >>>+u8 sunxi_sid_read_byte(const int key)
> >>>+{
> >>>+ u32 sid_key;
> >>>+ u8 ret;
> >>>+
> >>>+ ret = 0;
> >>>+ if (likely((key <= SUNXI_SID_SIZE))) {
> >>>+ sid_key = ioread32(p->sid_base + keys_lut[key >> 2]);
> >>>+ switch (key % 4) {
> >>>+ case 0:
> >>>+ ret = (sid_key >> 24) & 0xff;
> >>>+ break;
> >>>+ case 1:
> >>>+ ret = (sid_key >> 16) & 0xff;
> >>>+ break;
> >>>+ case 2:
> >>>+ ret = (sid_key >> 8) & 0xff;
> >>>+ break;
> >>>+ case 3:
> >>>+ ret = sid_key & 0xff;
> >>>+ break;
> >>>+ }
> >>>+ }
> >>
> >>Come on, you can do better. This lookup table is useless.
> >I didn't want to depend on the fixed layout of memory, but consider it
> >removed.
> But i'm not smart enough :p
>
> We can either use the look up table (which does have benefits as its
> potentially more future proof), or do some ((key >> 2) << 2) to
> 'drop' the LSB's that we want to ignore (unless there's some smarter
> way).
>
> Personally, I think the LUT is a little cleaner and more readable,
> but I guess if you look at poor efficiency, the lut costs some
> memory, the left/right shift cost an additional >> 2 ... what you
> prefer.
What about:
val = ioread32be(base + (key / 4));
val >>= (key % 4) * 8;
return val & 0xff;
No lookup table, no weird swich statement, and you get the endianness
conversion only when you need it.
Maxime
--
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
next prev parent reply other threads:[~2013-05-25 12:22 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-17 13:35 [PATCH 0/2] Driver for Allwinner sunxi Security ID Oliver Schinagl
2013-05-17 13:35 ` [PATCH 1/2] Initial support for Allwinner's Security ID fuses Oliver Schinagl
2013-05-17 13:45 ` Arnd Bergmann
2013-05-17 18:54 ` Oliver Schinagl
2013-05-17 21:18 ` Maxime Ripard
2013-05-18 17:19 ` Oliver Schinagl
2013-05-19 15:22 ` Maxime Ripard
2013-05-24 21:50 ` Oliver Schinagl
2013-05-25 12:22 ` Maxime Ripard [this message]
2013-05-25 19:25 ` Oliver Schinagl
2013-05-26 9:35 ` Maxime Ripard
2013-05-23 7:56 ` Linus Walleij
2013-05-23 8:10 ` Oliver Schinagl
2013-05-23 8:20 ` Linus Walleij
2013-05-23 14:58 ` Maxime Ripard
2013-05-23 15:05 ` Oliver Schinagl
2013-05-23 15:27 ` Maxime Ripard
2013-05-17 13:35 ` [PATCH 2/2] Add sunxi-sid to dts for sun4i and sun5i Oliver Schinagl
2013-05-17 21:21 ` Maxime Ripard
2013-06-02 14:58 [PATCH 0/2] v2 Driver for Allwinner sunxi Security ID Oliver Schinagl
2013-06-02 14:58 ` [PATCH 1/2] Initial support for Allwinner's Security ID fuses Oliver Schinagl
2013-06-02 15:09 ` Russell King - ARM Linux
2013-06-02 15:21 ` Oliver Schinagl
2013-06-06 19:16 ` Andy Shevchenko
2013-06-10 21:43 ` Oliver Schinagl
2013-06-11 10:51 ` Andy Shevchenko
2013-06-14 23:16 [PATCH 0/2] v3 Driver for Allwinner sunxi Security ID Oliver Schinagl
2013-06-14 23:16 ` [PATCH 1/2] Initial support for Allwinner's Security ID fuses Oliver Schinagl
2013-06-15 2:14 ` Andy Shevchenko
2013-06-15 9:34 ` Oliver Schinagl
2013-06-15 10:28 ` Tomasz Figa
2013-06-17 10:36 ` Oliver Schinagl
2013-06-17 11:25 ` Russell King - ARM Linux
2013-06-17 11:32 ` Oliver Schinagl
2013-06-17 11:51 ` Maxime Ripard
2013-06-17 12:04 ` Oliver Schinagl
2013-06-17 12:51 ` Tomasz Figa
2013-06-17 13:10 ` Oliver Schinagl
2013-06-17 13:23 ` Tomasz Figa
2013-06-17 13:47 ` Oliver Schinagl
2013-06-17 20:59 [PATCH 0/2] v4 Driver for Allwinner sunxi Security ID Oliver Schinagl
2013-06-17 20:59 ` [PATCH 1/2] Initial support for Allwinner's Security ID fuses Oliver Schinagl
2013-06-17 21:06 ` Tomasz Figa
2013-06-17 22:58 ` Greg KH
2013-06-24 9:29 ` Maxime Ripard
2013-06-24 16:04 ` Greg KH
2013-06-24 17:11 ` Oliver Schinagl
2013-06-24 18:15 ` Greg KH
2013-06-24 21:21 ` Oliver Schinagl
2013-06-24 21:46 ` Greg KH
2013-06-26 8:32 ` Oliver Schinagl
2013-06-26 17:51 ` Greg KH
2013-07-05 7:24 ` Oliver Schinagl
2013-07-06 19:36 ` Greg KH
2013-07-07 0:17 ` Greg KH
2013-06-26 9:10 ` Russell King - ARM Linux
2013-06-26 17:51 ` Greg KH
2013-06-24 21:04 ` Maxime Ripard
2013-06-26 9:22 ` Geert Uytterhoeven
2013-06-26 17:49 ` Greg KH
2013-06-18 5:41 ` Andy Shevchenko
2013-08-27 14:13 [PATCHv5 0/2] Driver for Allwinner sunxi Security ID oliver+list
2013-08-27 14:13 ` [PATCH 1/2] Initial support for Allwinner's Security ID fuses oliver+list
2013-08-27 15:42 ` Maxime Ripard
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