From: Gautham R Shenoy <ego@linux.vnet.ibm.com>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: "ego@linux.vnet.ibm.com" <ego@linux.vnet.ibm.com>,
Linux PM list <linux-pm@vger.kernel.org>,
"linuxppc-dev@ozlabs.org" <linuxppc-dev@ozlabs.org>
Subject: Re: [PATCH v3 5/5] powernv:cpufreq: Implement the driver->get() method
Date: Fri, 21 Mar 2014 19:55:55 +0530 [thread overview]
Message-ID: <20140321142555.GF2493@in.ibm.com> (raw)
In-Reply-To: <CAKohpomHHToqOND=ZtMeqBd11X3VA=PeZ2K6Tyj31y2V4G9-uA@mail.gmail.com>
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.
next prev parent reply other threads:[~2014-03-21 14:26 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-03-20 12:10 [PATCH v3 0/5] powernv: Enable Dynamic Frequency Gautham R. Shenoy
2014-03-20 12:10 ` [PATCH v3 1/5] powernv: cpufreq driver for powernv platform Gautham R. Shenoy
2014-03-21 8:41 ` Viresh Kumar
2014-03-21 10:43 ` Gautham R Shenoy
2014-03-21 10:54 ` Viresh Kumar
2014-03-21 11:40 ` Srivatsa S. Bhat
2014-03-21 13:23 ` Gautham R Shenoy
2014-03-21 13:34 ` Viresh Kumar
2014-03-21 14:54 ` Gautham R Shenoy
2014-03-22 3:43 ` Viresh Kumar
2014-03-21 14:48 ` Vaidyanathan Srinivasan
2014-03-22 3:43 ` Viresh Kumar
2014-03-21 11:45 ` Srivatsa S. Bhat
2014-03-21 11:47 ` Viresh Kumar
2014-03-20 12:10 ` [PATCH v3 2/5] powernv, cpufreq:Add per-core locking to serialize frequency transitions Gautham R. Shenoy
2014-03-21 6:24 ` Gautham R Shenoy
2014-03-21 8:42 ` Viresh Kumar
2014-03-21 9:56 ` Srivatsa S. Bhat
2014-03-20 12:10 ` [PATCH v3 3/5] powernv:cpufreq: Create pstate_id_to_freq() helper Gautham R. Shenoy
2014-03-21 6:24 ` Gautham R Shenoy
2014-03-20 12:10 ` [PATCH v3 4/5] powernv:cpufreq: Export nominal frequency via sysfs Gautham R. Shenoy
2014-03-21 8:47 ` Viresh Kumar
2014-03-21 9:55 ` Gautham R Shenoy
2014-03-21 9:57 ` Viresh Kumar
2014-03-20 12:11 ` [PATCH v3 5/5] powernv:cpufreq: Implement the driver->get() method Gautham R. Shenoy
2014-03-21 6:25 ` Gautham R Shenoy
2014-03-21 9:31 ` Viresh Kumar
2014-03-21 11:04 ` Gautham R Shenoy
2014-03-21 11:45 ` Viresh Kumar
2014-03-21 12:01 ` David Laight
2014-03-21 12:31 ` Gautham R Shenoy
2014-03-21 12:34 ` Viresh Kumar
2014-03-21 15:01 ` David Laight
2014-03-21 13:04 ` Gautham R Shenoy
2014-03-21 13:12 ` Viresh Kumar
2014-03-21 14:25 ` Gautham R Shenoy [this message]
2014-03-21 22:56 ` Benjamin Herrenschmidt
2014-03-22 7:53 ` Gautham R Shenoy
2014-03-21 7:46 ` [PATCH v3 0/5] powernv: Enable Dynamic Frequency Viresh Kumar
2014-03-21 8:19 ` Gautham R Shenoy
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