linux-kernel - Re: [PATCH 18/20] clocksource / acpi: Add macro CLOCKSOURCE_ACPI

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Date:	Fri, 24 Jan 2014 23:45:03 +0800
From:	Hanjun Guo <hanjun.guo@...aro.org>
To:	Mark Rutland <mark.rutland@....com>
CC:	Linus Walleij <linus.walleij@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Olof Johansson <olof@...om.net>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Rob Herring <robh@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Patch Tracking <patches@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>,
	Charles Garcia-Tobin <Charles.Garcia-Tobin@....com>,
	Amit Daniel Kachhap <amit.daniel@...sung.com>
Subject: Re: [PATCH 18/20] clocksource / acpi: Add macro CLOCKSOURCE_ACPI_DECLARE

Hi Mark,

On 2014年01月24日 20:32, Mark Rutland wrote:
> On Fri, Jan 24, 2014 at 12:12:24AM +0000, Hanjun Guo wrote:
>> Hi Linus,
>>
>> Sorry for the late reply.
>>
>> On 2014年01月22日 16:26, Linus Walleij wrote:
>>> On Fri, Jan 17, 2014 at 1:25 PM, Hanjun Guo <hanjun.guo@...aro.org> wrote:
>>>
>>>> From: Amit Daniel Kachhap <amit.daniel@...sung.com>
>>>>
>>>> This macro does the same job as CLOCKSOURCE_OF_DECLARE. The device
>>>> name from the ACPI timer table is matched with all the registered
>>>> timer controllers and matching initialisation routine is invoked.
>>>>
>>>> Signed-off-by: Amit Daniel Kachhap <amit.daniel@...sung.com>
>>>> Signed-off-by: Hanjun Guo <hanjun.guo@...aro.org>
>>> Actually I have a fat patch renaming CLOCKSOURCE_OF_DECLARE()
>>> to TIMER_OF_DECLARE() and I think this macro, if needed, should
>>> be named TIMER_ACPI_DECLARE().
>>>
>>> The reason is that "clocksource" is a Linux-internal name and this
>>> macro pertains to the hardware name in respective system
>>> description type.
>> That make sense to me too, I will update in next version if
>> this patch is still needed.
>>
>>>> +#ifdef CONFIG_ACPI
>>>> +#define CLOCKSOURCE_ACPI_DECLARE(name, compat, fn)                     \
>>>> +       static const struct acpi_device_id __clksrc_acpi_table_##name   \
>>>> +               __used __section(__clksrc_acpi_table)                   \
>>>> +                = { .id = compat,                              \
>>>> +                    .driver_data = (kernel_ulong_t)fn }
>>>> +#else
>>>> +#define CLOCKSOURCE_ACPI_DECLARE(name, compat, fn)
>>>> +#endif
>>> This hammers down the world to compile one binary for ACPI
>>> and one binary for device tree. Maybe that's fine, I don't know.
>> This is a problem we can have some discussion on it.
>> I prefer mutually exclusive ACPI and DT support.
> A lot of work has been put into making a single kernel boot everywhere.
> It's forced duplicated code to be factored out, and it's made the kernel
> more flexible. While it has been painful, it's forced a far higher
> quality standard across the board(s).
>
> Having a separate ACPI-capable or DT-capable kernels goes completely
> against that, and it's completely broken:
>
> * It doubles the testing effort required for a particular kernel. I can
>    guarantee that we will miss bugs (even amazingly bad build bugs)
>    because no-one will be able to test a full suite of kernels.
>
> * It introduces the possibility of completely pointles arbitrary
>    differences between the two. How long until we see the first bug-fix
>    that only works in one configuration?
>
> * It creates additional work for distributions, which need to build more
>    kernels test them, distribute them, and document which platforms which
>    kernels are supported on. This creates more pain for end-users too.
>    
> Eventually we _will_ get fed up with all of those, and we'll have to do
> painful invasive work to make the kernel decide at runtime.
>
> Having separate kernels is a lazy shortcut. It's painful for everyone,
> leads to a greater maintenance overhead, it's not what we want now and
> not what we want in future.
>
> No thanks.
>
> Either the kernel figures out whether or not to deal with ACPI at
> runtime, or it doesn't deal with it at all.

I fully agree with you for the single kernel image, I didn't notice this 
before, sorry for
my noise about the exclusive ACPI and DT support.

Thank you very much to let things much more clearer :)

Hanjun
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