Date: Tue, 18 Jun 2024 01:26:19 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Jean Delvare <jdelvare@...e.de>
Cc: Douglas Anderson <dianders@...omium.org>, 
	dri-devel@...ts.freedesktop.org, LKML <linux-kernel@...r.kernel.org>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Daniel Vetter <daniel@...ll.ch>, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH RESEND] drm/display: Drop obsolete dependency on
 COMPILE_TEST

On Mon, Jun 17, 2024 at 08:18:14PM GMT, Jean Delvare wrote:
> On Mon, 17 Jun 2024 14:55:22 +0300, Dmitry Baryshkov wrote:
> > On Mon, Jun 17, 2024 at 01:23:48PM GMT, Jean Delvare wrote:
> > > Hi Dmitry,
> > > 
> > > Thanks for your feedback.
> > > 
> > > On Mon, 17 Jun 2024 12:57:19 +0300, Dmitry Baryshkov wrote:  
> > > > On Mon, Jun 17, 2024 at 10:30:30AM GMT, Jean Delvare wrote:  
> > > > > Since commit 0166dc11be91 ("of: make CONFIG_OF user selectable"), it
> > > > > is possible to test-build any driver which depends on OF on any
> > > > > architecture by explicitly selecting OF. Therefore depending on
> > > > > COMPILE_TEST as an alternative is no longer needed.    
> > > > 
> > > > The goal of this clause is to allow build-testing the driver with OF
> > > > being disabled (meaning that some of OF functions are stubbed and some
> > > > might disappear). I don't see how user-selectable OF provides the same
> > > > result.  
> > > 
> > > Historically, the goal of this clause *was* to allow build-testing the
> > > driver on architectures which did not support OF, and that did make
> > > sense back then. As I understand it, building the driver without OF
> > > support was never a goal per se (if it was, then the driver wouldn't be
> > > set to depend on OF in the first place).
> > > 
> > > Some of my other submissions include the following explanation which
> > > you might find useful (I ended up stripping it on resubmission after
> > > being told I was being too verbose, but maybe it was needed after all):
> > > 
> > > It is actually better to always build such drivers with OF enabled,
> > > so that the test builds are closer to how each driver will actually be
> > > built on its intended target. Building them without OF may not test
> > > much as the compiler will optimize out potentially large parts of the
> > > code. In the worst case, this could even pop false positive warnings.
> > > Dropping COMPILE_TEST here improves the quality of our testing and
> > > avoids wasting time on non-existent issues.  
> > 
> > This doesn't seem to match the COMPILE_TEST usage that I observe in
> > other places. For example, we frequently use 'depends on ARCH_QCOM ||
> > COMPILE_TEST'. Which means that the driver itself doesn't make sense
> > without ARCH_QCOM, but we want for it to be tested on non-ARCH_QCOM
> > cases. I think the same logic applies to 'depends on OF ||
> > COMPILE_TEST' clauses. The driver (DP AUX bus) depends on OF to be fully
> > functional, but it should be compilable even without OF case.
> 
> The major difference is that one can't possibly enable ARCH_QCOM if
> building on X86 for example. Therefore COMPILE_TEST is the only way to
> let everyone (including randconfig/allmodconfig build farms) test-build
> your code.
> 
> On the other hand, if you want to test-build drm_dp_aux_bus, you can
> simply enable OF, because it is available on all architectures and
> doesn't depend on anything. No need for COMPILE_TEST.

I'd probably let Doug respond, what was his intention. 

> 
> For clarity, I'm not advocating against the use of COMPILE_TEST,
> actually if you check the history of my kernel contributions 10 years
> back, you'll find commits from me adding COMPILE_TEST in addition to
> arch-specific dependencies to many drivers. All I'm saying is that it
> should only be used when it is the only way to enable the build.


-- 
With best wishes
Dmitry

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