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Date: Fri, 21 Mar 2014 19:55:55 +0530
From: Gautham R Shenoy <ego@linux.vnet.ibm.com>
To: Viresh Kumar <viresh.kumar@linaro.org>
Subject: Re: [PATCH v3 5/5] powernv:cpufreq: Implement the driver->get() method
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On Fri, Mar 21, 2014 at 06:42:50PM +0530, Viresh Kumar wrote:
> On 21 March 2014 18:34, Gautham R Shenoy <ego@linux.vnet.ibm.com> wrote:
> > Consider the case when pmspr = 0x00feffffffffffff;
> >
> > We are interested in extracting the value 'fe'. And ensure that when
> > we store this value into an int, we get the sign extension right.
> >
> > So the following doesn't work:
> >
> >    pstate_id = (pmspr_val >> 48) & 0xFFFFFFFF;
> 
> What about pstate_id = (pmspr_val >> 48) & 0xFF; ??

Nope. 

Suppose the pmspr_val is contained in the register 
%rax, and pstate_id corresponds to the address -20(%rbp) then:

	pstate_id = (pmspr_val >> 48) & 0xFF;

would corrspond to 

        shrq    $48, %rax        // Left shift by 48 bits
        andl    $255, %eax       // Mask the lower 32 bits of %rax with 0x000000FF
        movl    %eax, -20(%rbp)  // Store the lower 32 bits of %rax
                                    into pstate_id


On the other hand, 

   pstate_id = (int)((s8)((pmspr_val >> 48) & 0xFF));

would correspond to:

        shrq    $48, %rax  // Left shift by 48 bits.
        movsbl  %al, %eax  // Move the lower 8 bits of %rax to %eax
                              with sign-extension. 
        movl    %eax, -20(%rbp) // store the result in pstate_id;

So with this, we are getting the sign extension due to the use of movsbl.

And if local_pstate_id corresponds to the address -1(%rbp) then:

       local_pstate_id = (pmspr_val >> 48) & 0xFF;
       pstate_id = local_pstate_id;

would correspond to:

        shrq    $48, %rax        // Left shift by 48 bits
        movb    %al, -1(%rbp)    //copy the lower 8 bits to local_pstate_id
        movsbl  -1(%rbp), %eax   //move the contents of
                                   local_pstate_id to %eax with sign extension.
        movl    %eax, -20(%rbp)  // Store the result in pstate_id


Hope this helps :-)

--
Thanks and Regards
gautham.