Messages in this thread | | | Date | Sat, 25 May 2013 14:22:08 +0200 | From | Maxime Ripard <> | Subject | Re: [PATCH 1/2] Initial support for Allwinner's Security ID fuses |
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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
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