Re: writing to a floating point register ?

[Sven Luther <luther@dpt-info.u-strasbg.fr>]

On Wed, Feb 05, 2003 at 11:28:58AM +0100, Geert Uytterhoeven wrote:
> On Wed, 5 Feb 2003, Sven Luther wrote:
> > while writing a fbdev driver, i need to write a value to a floating
> > point register. Do we have any macro doing this, or should i need to
> > calculate the sign, mantissa and exponent by hand ?
> > 
> > Just doing a unsigned int cast would round the value i think, and thus
> > not give the right result.
> 
> You cannot use floating point math in the kernel.
> 
> What exactly are you trying to achieve?

Well, my framebuffer is not linear, and i have to enable a bypass unit
to fake a linear framebuffer. This bypass unit need that i write the
bytestride/64 32bit floating point value in a register.

for example : 1024 in 32 bpp => 4*1024/64 = 64.

64 is 0100 0000 or 1.0 * 2^6.

So i have sign = 0, exp = 127+6=133 = 10000101, and mantissa = 0.

which gives 0100 0010 1000 0000 ... or 0x42 80 00 00.

I was hoping that i would not need to do this calculation by hand, but i
guess it is not possible, since like you said, we cannot use the FP
unit in the kernel.

Or maybe there are some convenient macros available for that or
something ?

Friendly,

Sven Luther


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