Re: [PATCH v8 01/17] spi: add basic support for SPI offloading

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Tue, Feb 11, 2025 at 07:45:30PM +0100, Uwe Kleine-König wrote:
> Hello Andy,
> 
> On Tue, Feb 11, 2025 at 04:35:34PM +0200, Andy Shevchenko wrote:
> > On Tue, Feb 11, 2025 at 04:31:45PM +0200, Andy Shevchenko wrote:
> > > On Tue, Feb 11, 2025 at 04:29:33PM +0200, Andy Shevchenko wrote:
> > > > On Tue, Feb 11, 2025 at 04:20:50PM +0200, Andy Shevchenko wrote:
> > > > > On Tue, Feb 11, 2025 at 01:00:08PM +0000, Mark Brown wrote:
> > > > > > On Mon, Feb 10, 2025 at 10:33:31PM +0200, Andy Shevchenko wrote:
> > > > > > > On Mon, Feb 10, 2025 at 05:48:00PM +0000, Mark Brown wrote:
> > > > > > > > On Mon, Feb 10, 2025 at 11:11:23AM -0600, David Lechner wrote:
> > > > > > 
> > > > > > > > > In this case, we specifically split up the headers so that the only time you
> > > > > > > > > would ever include this header is if you need to call functions in this
> > > > > > > > > namespace (i.e. struct definitions are in linux/spi/offload/types.h which
> > > > > > > > > doesn't import the namespace). So this doesn't actually seem like a problem
> > > > > > > > > to me.
> > > > > > 
> > > > > > > > Indeed - I can't see any case where a user would need the header without
> > > > > > > > needing the namespace.
> > > > > > 
> > > > > > > You are looking from the other end. What I'm telling is that anyone who adds
> > > > > > > a header, automatically gets a namespace. What's the point to have namespace
> > > > > > > if it won't easily prevent from (ab)using it in the code. I consider putting
> > > > > > > MODULE_IMPORT_NS() in the headers a bit weird.
> 
> There was a similar discussion some time ago about the lpss pwm driver
> (https://lore.kernel.org/linux-pwm/Z09YJGifvpENYNPy@smile.fi.intel.com/).
> The arguments that you didn't accept back then already are similar to
> the ones that were brought forward here.
> The TL;DR; is: Adding MODULE_IMPORT_NS() to a header makes it easier for
> code to use the exported symbols. Yes, that includes abusers of the
> code.
> 
> But if you mostly care about the regular users of an API/ABI, making
> things easy for those is the thing that matters. Agreed, if you think
> that module namespaces are primarily a line of defence against abusers,
> adding the import to the header weakens that defence (a bit). However a
> typical header includes function prototypes and macros. Those also make
> it easier for abusers. With your argumentation we better don't create
> headers at all?
> 
> There are other benefits of module namespaces like reducing the set of
> globally available symbols which speeds up module loading or the
> ability to see in the module meta data that a namespace is used.

Thank you for summarizing the previous discussion.

> > > > > > Sure, but there's no case where anyone should ever be adding the header
> > > > > > without adding the namespace which does rather sound like the sort of
> > > > > > thing where you should just move the namespace addition to the header.
> > > > > 
> > > > > $ git grep -lw MODULE_IMPORT_NS | wc -l
> > > > > 651
> > > > > 
> > > > > $ git grep -lw MODULE_IMPORT_NS | grep '\.h$'
> > > > > 
> > > > > drivers/base/firmware_loader/sysfs.h
> > > > > drivers/iio/adc/ltc2497.h
> > > > > drivers/pwm/pwm-dwc.h
> > > > > ^^^ These ones are probably fine as they are not in include/
> > > > > 
> > > > > include/kunit/visibility.h
> > > > > include/linux/module.h
> > > > > include/linux/pwm.h
> > > > > 
> > > > > I believe these three are misuses of MODULE_IMPORT_NS(). Because one may add
> > > > 
> > > > _Two_, of course, module.h provides the macro :-)
> > > 
> > > And after looking into include/kunit/visibility.h it becomes only a single one.
> > > So, PWM is abuser of MODULE_IMPORT_NS() and this series added one more.
> > 
> > > > > a header just as a "proxy" one (copy'n'paste, for example) and we know that is
> > > > > real as we saw a lot of code that has semi-random header inclusion blocks.
> > 
> > And thinking of more realistic example when we want header and do *not* want a
> > namespace is the simple use of the macro / or data type from it without
> > actually relying on the APIs.
> 
> The problem of your more realistic example is that it doesn't apply
> here. A user of include/linux/pwm.h (or the header under discussion
> here) won't only use a macro or two and so not benefit from the imported
> module namespace.

It may not apply _today_, but it may be applicable tomorrow as headers are tend
to grow and use another headers and so on.

> Nobody intends to import all possible namespaces in <linux/kernel.h>.
> 
> > So, in case of the header structure like
> > 
> > foo_constants.h
> > foo_types.h
> > foo_api.h
> > foo_uplevel_something.h
> > 
> > The MODULE_IMPORT_NS() would make sense only to foo_api.h. And I still would
> > question that. As I explained that header may simply become a stale one or
> > being used by a mistake.
> 
> I have no problem here. If the header becomes stale we will most
> probably notice that eventually and remove it.

Lol. Look at the header hell we have now. 98% code in the drivers/ just show
that the developers either don't care or do not understand C (in terms of
what headers are for and why it's important to follow IWYU principle).

> Maybe the unused namespace even makes it easier to spot that issue.

Do we have an existing tools for that?

> See
> https://lore.kernel.org/r/20250123103939.357160-2-u.kleine-koenig@baylibre.com
> for an example which I found exactly like that.


-- 
With Best Regards,
Andy Shevchenko