From: Oliver Schinagl <oliver+list@schinagl.nl>
To: Maxime Ripard <maxime.ripard@free-electrons.com>
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 21:25:51 +0200 [thread overview]
Message-ID: <51A1103F.6000702@schinagl.nl> (raw)
In-Reply-To: <20130525122208.GD17847@lukather>
On 05/25/13 14:22, Maxime Ripard wrote:
> 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.
Ok I think I like the Endianess, ioread32be does the right thing then?
I'll read up on that.
As for key / 4; how will that work without the lut?
Lets take byte 14 (out of the available 16). Byte 14 (0x0e) is located
in SID_KEY3, so base + 0x0c. We need to write a whole 32bit word to keep
with alignment, the registers go wakko if you do unaligned reads. So we
need to read the entire 32 bits, then find our byte.
key / 4 for 14 yields 0x03. So we have base + 0x03, which is not what we
want. We want base + 0x0c, which is either ((key >> 2) << 2)) Or, ((key
/ 4) * 4)) which to me, are both equally confusing. So we either use the
look up table, which is a little cleaner and is a bit more future proof
if either a) there's more 'eeprom like' storage which can be combined
herein or b) 'a' next version has non-continuing regions. Granted
neither is something to worry about right now.
Turl already mentioned the calculated shift, instead of the switch. I
agree to also like it better and have already rewritten that bit.
If I made a really stupid thinking mistake or my math is somehow wrong,
feel free to point it out :) I don't mind manning up to my mistakes :)
>
> Maxime
>
next prev parent reply other threads:[~2013-05-25 19:25 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
2013-05-25 19:25 ` Oliver Schinagl [this message]
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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