linux-kernel - Re: [PATCH] ARM: cache-l2x0.c: save aux ctrl for resume in case that l2x0 is enabled before init

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Message-ID: <20120426105651.GB6294@arm.com>
Date:	Thu, 26 Apr 2012 11:56:51 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Yilu Mao <ylmao@...vell.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"Baohua.Song@....com" <Baohua.Song@....com>,
	"santosh.shilimkar@...com" <santosh.shilimkar@...com>,
	"robherring2@...il.com" <robherring2@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Ning Jiang <njiang1@...vell.com>
Subject: Re: [PATCH] ARM: cache-l2x0.c: save aux ctrl for resume in case
 that l2x0 is enabled before init

On Thu, Apr 26, 2012 at 11:48:18AM +0100, Yilu Mao wrote:
> On 04/26/2012 06:38 PM, Catalin Marinas wrote:
> > On Thu, Apr 26, 2012 at 11:35:39AM +0100, Yilu Mao wrote:
> >> On 04/26/2012 06:28 PM, Catalin Marinas wrote:
> >>> On Thu, Apr 26, 2012 at 11:09:26AM +0100, Yilu Mao wrote:
> >>>> On 04/26/2012 05:44 PM, Catalin Marinas wrote:
> >>>>> On Thu, Apr 26, 2012 at 10:25:31AM +0100, Yilu Mao wrote:
> >>>>>> On 04/26/2012 04:35 PM, Catalin Marinas wrote:
> >>>>>>> On Thu, Apr 26, 2012 at 06:00:09AM +0100, Yilu Mao wrote:
> >>>>>>>> On 04/24/2012 04:28 PM, Catalin Marinas wrote:
> >>>>>>>>> On Tue, Apr 24, 2012 at 03:41:20AM +0100, Yilu Mao wrote:
> >>>>>>>>>> +	l2x0_saved_regs.aux_ctrl = aux;
> >>>>>>>>>> +
> >>>>>>>>>>       	aux&= aux_mask;
> >>>>>>>>>>       	aux |= aux_val;
> >>>>>>>>>
> >>>>>>>>> I think that's the wrong place to save it, it should be after the
> >>>>>>>>> masking was done.
> >>>>>>>>>
> >>>>>>>>> Anyway, if we cannot write this register in l2x0_init() because the L2
> >>>>>>>>> was enabled, do we expect the L2 to be disabled during resume?
> >>>>>>>>>
> >>>>>>>> Sorry, I don't think so.
> >>>>>>>> This is the right place to save it because we must make sure the saved
> >>>>>>>> aux_ctrl is the same as what it is set.
> >>>>>>>> If we save it after masking was done, the saved value will be different
> >>>>>>>> because we can't actually change the real setting.
> >>>>>>>
> >>>>>>> And since we can't actually change the real setting on the resume path,
> >>>>>>> why do we need to save it anyway. Is your L2 cache disabled on the
> >>>>>>> resume path but not on the cold boot one?
> >>>>>>
> >>>>>> We can't change L2 aux ctrl setting when do init because it has been
> >>>>>> enabled.
> >>>>>
> >>>>> This is normally for the case where the kernel running in non-secure
> >>>>> mode is not allowed to write the L2 aux ctrl register. Does this
> >>>>> permission change with core idle?
> >>>>>
> >>>> Yes, your understanding of previous mail is right. The L2 is enabled on
> >>>> code boot and it is disabled on the resume in our case.
> >>>
> >>> But the kernel either runs in secure mode or the non-secure access to
> >>> this register is allowed.
> >>>
> >>>> So if we don't have such patch, when core idle exit, L2 cache aux ctrl
> >>>> register will be set to 0x0 because l2x0_saved_regs.aux_ctrl is not
> >>>> initialized.
> >>>
> >>> You could still make sure that the mask passed doesn't affect the
> >>> original setting and save it after masking.
> >>>
> >> Do you mean the code is like this:
> >> aux = readl_relaxed(l2x0_base + L2X0_AUX_CTRL);
> >> aux&= aux_mask;
> >> l2x0_saved_regs.aux_ctrl = aux;
> >>
> >> Then the saved value is not the same as real setting. So the restored
> >> value after core idle will not the same as before... This is not what we
> >> expected.
> >
> > My point was that on your platform you pass an aux_mask that is meant to
> > change the already set aux_ctlr value. Why do you pass such mask to be
> > anything other than ~0UL in this case?
> >
> Sorry I still can't catch your point...
> In our platform, we actually use ~0UL as aux mask.
> 
> Anyway, the two arguments, aux_value and aux_mask, are both for changing 
> the original value set before kernel bootup. If L2 cache is enabled in 
> cold boot, both two arguments are useless because we can't set the 
> register. So we must make sure we can restore the original value after 
> core idle.

So if your platform passes aux_mask = ~0UL and aux_val = 0, there is no
change to the read aux_ctlr value.

If for whatever reason (same binary running in different configuration)
you need to pass the aux_mask and aux_val different from the above, then
your argument makes sense. But I just want to be clear.

-- 
Catalin
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