What's logic behind this?

[lilzz]

Code: Select all

 cfg = *(&pio->cfg[0] + index);
    cfg &= ~(0xf << offset);
    cfg |= val << offset;

    *(&pio->cfg[0] + index) = cfg

anyone see the logic of this?

[ShiftPlusOne]

You're clearing a bit then setting it to val.

Again, context please. Where's it from? What's the hardware? Do you have a datasheet? Google tells me it's for olimex hardware or some other allwinner board, in which case I think you best ask them.

[mikronauts]

Code: Select all

 cfg = *(&pio->cfg[0] + index);
    cfg &= ~(0xf << offset);
    cfg |= val << offset;

    *(&pio->cfg[0] + index) = cfg

anyone see the logic of this?

The first line gets the content of a configuration register

Second line clears a nibble in the configuration register

Third line places a new value in the configuration register (where the nibble was cleared)

Last line writes it back.

Looks like:

index = specifies which index register to modify
&pio->cfg[0] = base address of a range of configuration registers
offset = bit offset for the nibble to be modified

Fairly normal way of modifying a memory mapped configuration register.

[ShiftPlusOne]

Ah, sorry for saying bit... missed the 0xf

[Heater]

mikronauts,

Fairly normal way of modifying a memory mapped configuration register.

Normal?!

Why not just write:

Code: Select all

cfg = pio->cfg[index];

Which compiles to the same code as shown above but is a lot clearer.

The code as given is only normal in that it is how some C programmers obfuscate their code for job security

[mikronauts]

I've seen worse, including written as answers to questions given to job applicants (needless to say they were not hired)

At least it indicated a minimal understanding of pointers.

I did not re-write the code, as that is not what the OP asked for.

mikronauts,

Fairly normal way of modifying a memory mapped configuration register.

Normal?!

Why not just write:

Code: Select all

cfg = pio->cfg[index];

Which compiles to the same code as shown above but is a lot clearer.

The code as given is only normal in that it is how some C programmers obfuscate their code for job security

[Heater]

I've probably written worse myself

It's not clear to me what lilzz asked in the OP. As someone said, without context who knows what that code has any logic. I thought perhaps that weird addressing was a stumbling block for lilzz.

[mikronauts]

I thought lilzz did not understand what the code was doing... but I could be wrong

Heater, I cannot see you writing anything as ugly as that... never mind worse! ZOG would get you!

[Heater]

Mickronauts,

Indeed, ZOG would not approve.

How long does it take Parallax to swap a server? I'm getting twitchy here.

[PiGraham]

It looks like a test. Is it homework on pointers and bitwise logic?

[Ravenous]

It looks like a test. Is it homework on pointers and bitwise logic?

Looks like a psychiatrist's test for ill programmers to me

Like a Rorschach ink blot test for programmers

[mikronauts]

It is taking them too long...

I wonder if any content will be lost? That would suck.

Mickronauts,

Indeed, ZOG would not approve.

How long does it take Parallax to swap a server? I'm getting twitchy here.

[mikronauts]

Good point. I did not think of that.

It looks like a test. Is it homework on pointers and bitwise logic?

[mikronauts]

LOL!

It looks like a test. Is it homework on pointers and bitwise logic?

Looks like a psychiatrist's test for ill programmers to me

Like a Rorschach ink blot test for programmers

[PiGraham]

Like a Rorschach ink blot test for programmers

Could be. It's not a good programming style example!

[Heater]

Sure enough google finds that code in some Allwinner gpio library here: https://www.olimex.com/forum/index.php?topic=436.0

In a slightly different way:

Code: Select all

//   cfg = readl(&pio->cfg[0] + index);
   cfg = *(&pio->cfg[0] + index);
   cfg &= ~(0xf << offset);
   cfg |= val << offset;

//   writel(cfg, &pio->cfg[0] + index);
   *(&pio->cfg[0] + index) = cfg;

Seems to have been extracted from U-BOOT according to the comments.

It's code like that that leads us to Heartbleed and such problems.