linux-kernel - Re: [PATCH 14/19] gpu: drm: selftests: test-drm_dp_mst_helper: Place 'struct drm_dp_sideband_msg_req

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Date:   Mon, 9 Nov 2020 19:03:24 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     David Airlie <airlied@...ux.ie>,
        David Francis <David.Francis@....com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 14/19] gpu: drm: selftests: test-drm_dp_mst_helper: Place
 'struct drm_dp_sideband_msg_req_body' onto the heap

On Mon, Nov 09, 2020 at 04:40:04PM +0000, Lee Jones wrote:
> On Mon, 09 Nov 2020, Ville Syrjälä wrote:
> 
> > On Mon, Nov 09, 2020 at 04:12:58PM +0000, Lee Jones wrote:
> > > On Mon, 09 Nov 2020, Ville Syrjälä wrote:
> > > 
> > > > On Thu, Nov 05, 2020 at 02:45:12PM +0000, Lee Jones wrote:
> > > > > The stack is too full.
> > > > > 
> > > > > Fixes the following W=1 kernel build warning(s):
> > > > > 
> > > > >  drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c: In function ‘sideband_msg_req_encode_decode’:
> > > > >  drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c:161:1: warning: the frame size of 1176 bytes is larger than 1024 bytes [-Wframe-larger-than=]
> > > > > 
> > > > > Cc: David Airlie <airlied@...ux.ie>
> > > > > Cc: Daniel Vetter <daniel@...ll.ch>
> > > > > Cc: Lyude Paul <lyude@...hat.com>
> > > > > Cc: David Francis <David.Francis@....com>
> > > > > Cc: dri-devel@...ts.freedesktop.org
> > > > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > > > > ---
> > > > >  .../drm/selftests/test-drm_dp_mst_helper.c    | 29 ++++++++++++-------
> > > > >  1 file changed, 18 insertions(+), 11 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c b/drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c
> > > > > index 1d696ec001cff..0a539456f6864 100644
> > > > > --- a/drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c
> > > > > +++ b/drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c
> > > > > @@ -120,44 +120,51 @@ sideband_msg_req_equal(const struct drm_dp_sideband_msg_req_body *in,
> > > > >  static bool
> > > > >  sideband_msg_req_encode_decode(struct drm_dp_sideband_msg_req_body *in)
> > > > >  {
> > > > > -	struct drm_dp_sideband_msg_req_body out = {0};
> > > > > +	struct drm_dp_sideband_msg_req_body *out;
> > > > 
> > > > How big is it? And why is it that big?
> > > 
> > > It's a struct of a union of structs.
> > > 
> > > And it's all allocated on the stack.  Bad news!
> > 
> > That doesn't answer my questions.
> 
> It answers the second question.

Not really. A combination of structs and unions could be
pretty much any size.

> 
> The answer to the first question you can `grep` for yourself. ;)

I would rather run pahole on it. But why would you require
reviewers to jump through such extra hoops when you could
just put that information into the commit message? With that
anyone could review this without having to do a lot of extra
work.

-- 
Ville Syrjälä
Intel