Re: [PATCH 18/20] clocksource / acpi: Add macro CLOCKSOURCE_ACPI_DECLARE

[Hanjun Guo]

Hi Mark,

On 2014å01æ24æ 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 2014å01æ22æ 16:26, Linus Walleij wrote:
> > > On Fri, Jan 17, 2014 at 1:25 PM, Hanjun Guo <hanjun.guo@xxxxxxxxxx> wrote:
> > >
> > > > From: Amit Daniel Kachhap <amit.daniel@xxxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxxx>
> > > > Signed-off-by: Hanjun Guo <hanjun.guo@xxxxxxxxxx>
> > > 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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