Date:	Fri, 19 Feb 2016 02:58:10 +0000
From:	"Zheng, Lv" <lv.zheng@...el.com>
To:	Peter Hurley <peter@...leysoftware.com>,
	Aleksey Makarov <aleksey.makarov@...aro.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>
CC:	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King <linux@....linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J . Wysocki" <rjw@...ysocki.net>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Graeme Gregory <graeme.gregory@...aro.org>,
	"Al Stone" <ahs3@...hat.com>,
	Christopher Covington <cov@...eaurora.org>,
	"Len Brown" <lenb@...nel.org>
Subject: RE: [PATCH v3 1/5] ACPI: change __init to __ref for
 early_acpi_os_unmap_memory()

Hi,

> From: Peter Hurley [mailto:peter@...leysoftware.com]
> Subject: Re: [PATCH v3 1/5] ACPI: change __init to __ref for
> early_acpi_os_unmap_memory()
> 
> On 02/17/2016 07:36 PM, Zheng, Lv wrote:
> > Hi,
> >
> >> From: Aleksey Makarov [mailto:aleksey.makarov@...aro.org]
> >> Subject: Re: [PATCH v3 1/5] ACPI: change __init to __ref for
> >> early_acpi_os_unmap_memory()
> >>
> >> Hi Lv,
> >>
> >> Thank you for review.
> >>
> >> On 02/17/2016 05:51 AM, Zheng, Lv wrote:
> >>
> >> [..]
> >>
> >>>>> early_acpi_os_unmap_memory() is marked as __init because it calls
> >>>>> __acpi_unmap_table(), but only when acpi_gbl_permanent_mmap is not
> >> set.
> >>>>>
> >>>>> acpi_gbl_permanent_mmap is set in __init acpi_early_init()
> >>>>> so it is safe to call early_acpi_os_unmap_memory() from anywhere
> >>>>>
> >>>>> We need this function to be non-__init because we need access to
> >>>>> some tables at unpredictable time--it may be before or after
> >>>>> acpi_gbl_permanent_mmap is set.  For example, SPCR (Serial Port
> Console
> >>>>> Redirection) table is needed each time a new console is registered.
> >>>>> It can be quite early (console_initcall) or when a module is inserted.
> >>>>> When this table accessed before acpi_gbl_permanent_mmap is set,
> >>>>> the pointer should be unmapped.  This is exactly what this function
> >>>>> does.
> >>>> [Lv Zheng]
> >>>> Why don't you use another API instead of
> early_acpi_os_unmap_memory()
> >> in
> >>>> case you want to unmap things in any cases.
> >>>> acpi_os_unmap_memory() should be the one to match this purpose.
> >>>> It checks acpi_gbl_ppermanent_mmap in acpi_os_unmap_iomem().
> >>
> >> As far as I understand, there exist two steps in ACPI initialization:
> >>
> >> 1. Before acpi_gbl_permanent_mmap is set, tables received with
> >> acpi_get_table_with_size()
> >>    are mapped by early_memremap().  If a subsystem gets such a pointer it
> >> should be unmapped.
> >>
> >> 2  After acpi_gbl_permanent_mmap is set this pointer should not be
> unmapped
> >> at all.
> >>
> > [Lv Zheng]
> > This statement is wrong, this should be:
> > As long as there is a __reference__ to the mapped table, the pointer should
> not be unmapped.
> > In fact, we have a series to introduce acpi_put_table() to achieve this.
> > So your argument is wrong from very first point.
> >
> >> That exactly what early_acpi_os_unmap_memory() does--it checks
> >> acpi_gbl_permanent_mmap.
> >> If I had used acpi_os_unmap_memory() after acpi_gbl_permanent_mmap
> had
> >> been set,
> >> it would have tried to free that pointer with an oops (actually, I checked that
> >> and got that oops).
> >>
> >> So using acpi_os_unmap_memory() is not an option here, but
> >> early_acpi_os_unmap_memory()
> >> match perfectly.
> > [Lv Zheng]
> > I don't think so.
> > For definition block tables, we know for sure there will always be references,
> until "Unload" opcode is invoked by the AML interpreter.
> > But for the data tables, OSPMs should use them in this way:
> > 1. map the table
> > 2. parse the table and convert it to OS specific structures
> > 3. unmap the table
> > This helps to shrink virtual memory address space usages.
> >
> > So from this point of view, all data tables should be unmapped right after
> being parsed.
> > Why do you need the map to be persistent in the kernel address space?
> > You can always map a small table, but what if the table size is very big?
> >
> >>
> >>>> And in fact early_acpi_os_unmap_memory() should be removed.
> >>
> >> I don't think so -- I have explained why.  It does different thing.
> >> Probably it (and/or other functions in that api) should be renamed.
> >>
> > [Lv Zheng]
> > Just let me ask one more question.
> > eary_acpi_os_unmap_memory() is not used inside of ACPICA.
> > How ACPICA can work with just acpi_os_unmap_memory()?
> > You can check drivers/acpi/tbxxx.c.
> > Especially: acpi_tb_release_temp_table() and the code invoking it.
> >
> >>> [Lv Zheng]
> >>> One more thing is:
> >>> If you can't switch your driver to use acpi_os_unmap_memory() instead of
> >> early_acpi_os_unmap_memory(),
> >>> then it implies that your driver does have a defect.
> >>
> >> I still don't understand what defect, sorry.
> > [Lv Zheng]
> > If you can't ensure this sequence for using the data tables:
> > 1. map the table
> > 2. parse the table and convert it to OS specific structure
> > 3. unmap the table
> > It implies there is a bug in the driver or a bug in the ACPI subsystem core.
> 
> Exactly.
[Lv Zheng] 
So it looks to me:
Changing __init to __ref here is entirely not acceptable.
This API should stay being invoked during early stage.
Otherwise, it may leave us untrackable code that maps tables during early stage and leaks maps to the late stage.
If Linux contains such kind of code, I'm afraid, it will become impossible to introduce acpi_put_table() to clean up the mappings.
Because when acpi_put_table() is called during the late stage to free a map acquired during the early stage, it then obviously will end up with panic.

Thanks and best regards
-Lv

> The central problem here is the way Aleksey is trying to hookup a console.
> 
> What should be happening in this series is:
> 1. early map SPCR
> 2. parse the SPCR table
> 3. call add_preferred_console() to add the SPCR console to the console table
> 4. unmap SPCR
> 
> This trivial and unobtrusive method would already have a 8250 console
> running via SPCR. I've already pointed this out in previous reviews.
> 
> Further, the above method *will be required anyway* for the DBG2 table to
> start an earlycon, which I've already pointed out in previous reviews.
> 
> Then to enable amba-pl011 console via ACPI, add a console match() method
> similar to the 8250 console match() method, univ8250_console_match().
> 
> FWIW, PCI earlycon + console support was already submitted once before but
> never picked up by GregKH. I think I'll just grab that and re-submit so
> you would know what to emit for console options in the
> add_preferred_console().
> 
> 
> Regards,
> Peter Hurley
> 
> 
> >>> There is an early bootup requirement in Linux.
> >>> Maps acquired during the early stage should be freed by the driver during
> the
> >> early stage.
> >>> And the driver should re-acquire the memory map after booting.
> >>
> >> Exactly.  That's why I use early_acpi_os_unmap_memory().  The point is that
> >> that code can be
> >> called *before* acpi_gbl_permanent_mmap is set (at early initialization of
> for
> >> example 8250 console)
> >> or *after* acpi_gbl_permanent_mmap is set (at insertion of a module that
> >> registers a console),
> >> We just can not tell if the received table pointer should or sould not be freed
> >> with early_memunmap()
> >> (actually __acpi_unmap_table() or whatever) without checking
> >> acpi_gbl_permanent_mmap,
> >> but that's all too low level.
> > [Lv Zheng]
> > The driver should make sure that:
> > Map/unmap is paired during early stage.
> > For late stage, it should be another pair of map/unmap.
> >
> >>
> >> Another option, as you describe, is to get this pointer early, don't free it
> > [Lv Zheng]
> > I mean you should free it early.
> >
> >> untill acpi_gbl_permanent_mmap is set, then free it (with
> >> early_acpi_os_unmap_memory(), that's
> >> ok because acpi_gbl_permanent_mmap is set in an init code), then get the
> >> persistent
> >> pointer to the table.  The problem with it is that we can not just watch
> >> acpi_gbl_permanent_mmap
> >> to catch this exact moment.  And also accessing acpi_gbl_permanent_mmap
> is
> >> not good as it probably is
> >> an implementation detail (i. e. too lowlevel) of the ACPI code.
> >> Or I probably miss what you are suggesting.
> >>
> > [Lv Zheng]
> > I mean, you should:
> > During early stage:
> > acpi_os_map_memory()
> > Parse the table.
> > acpi_os_unmap_memory().
> >
> >>> This is because, during early bootup stage, there are only limited slots
> >> available for drivers to map memory.
> >>> So by changing __init to __ref here, you probably will hide many such
> defects.
> >>
> >> What defects?  This funcions checks acpi_gbl_permanent_mmap.
> >> BTW, exactly the same thing is done in the beginning of
> >> acpi_os_unmap_memory() and than's ok,
> >> that function is __ref.
> >>
> >>> And solving issues in this way doesn't seem to be an improvement.
> >>
> >> Could you please tell me where I am wrong?  I still don't understand your
> point.
> > [Lv Zheng]
> > But anyway, the defect should be in ACPI subsystem core.
> > The cause should be the API itself - acpi_get_table().
> >
> > So I agree you can use early_acpi_os_unmap_memory() during the period the
> root causes are not cleaned up.
> > But the bottom line is: the driver need to ensure that
> early_acpi_os_unmap_memory() is always invoked.
> > As long as you can ensure this, I don't have objections for deploying
> early_acpi_os_unmap_memory()  for now.
> >
> > Thanks and best regards
> > -Lv
> >
> >>
> >> Thank you
> >> Aleksey Makarov
> >>
> >>>
> >>> Thanks and best regards
> >>> -Lv
> >>>
> >>>>
> >>>> Thanks and best regards
> >>>> -Lv
> >>>>
> >>>>>
> >>>>> Signed-off-by: Aleksey Makarov <aleksey.makarov@...aro.org>
> >>>>> ---
> >>>>>  drivers/acpi/osl.c | 6 +++++-
> >>>>>  1 file changed, 5 insertions(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c
> >>>>> index 67da6fb..8a552cd 100644
> >>>>> --- a/drivers/acpi/osl.c
> >>>>> +++ b/drivers/acpi/osl.c
> >>>>> @@ -497,7 +497,11 @@ void __ref acpi_os_unmap_memory(void *virt,
> >>>>> acpi_size size)
> >>>>>  }
> >>>>>  EXPORT_SYMBOL_GPL(acpi_os_unmap_memory);
> >>>>>
> >>>>> -void __init early_acpi_os_unmap_memory(void __iomem *virt,
> acpi_size
> >>>> size)
> >>>>> +/*
> >>>>> + * acpi_gbl_permanent_mmap is set in __init acpi_early_init()
> >>>>> + * so it is safe to call early_acpi_os_unmap_memory() from anywhere
> >>>>> + */
> >>>>> +void __ref early_acpi_os_unmap_memory(void __iomem *virt,
> acpi_size
> >>>> size)
> >>>>>  {
> >>>>>  	if (!acpi_gbl_permanent_mmap)
> >>>>>  		__acpi_unmap_table(virt, size);
> >>>>> --
> >>>>> 2.7.1
> >>>>>
> >>>>> --
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> >>>>> the body of a message to majordomo@...r.kernel.org
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> >>>> --
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