Re: [PATCH] drm/i915/fbc: Avoid full proxy f_ops for FBC debug attributes

[Deepak R Varma]

On Tue, Dec 27, 2022 at 12:13:56PM -0500, Rodrigo Vivi wrote:
> On Tue, Dec 27, 2022 at 01:30:53PM +0530, Deepak R Varma wrote:
> > Using DEFINE_SIMPLE_ATTRIBUTE macro with the debugfs_create_file()
> > function adds the overhead of introducing a proxy file operation
> > functions to wrap the original read/write inside file removal protection
> > functions. This adds significant overhead in terms of introducing and
> > managing the proxy factory file operations structure and function
> > wrapping at runtime.
> > As a replacement, a combination of DEFINE_DEBUGFS_ATTRIBUTE macro paired
> > with debugfs_create_file_unsafe() is suggested to be used instead.  The
> > DEFINE_DEBUGFS_ATTRIBUTE utilises debugfs_file_get() and
> > debugfs_file_put() wrappers to protect the original read and write
> > function calls for the debug attributes. There is no need for any
> > runtime proxy file operations to be managed by the debugfs core.
> >
> > This Change is reported by the debugfs_simple_attr.cocci Coccinelle
> > semantic patch.
>
> I just checked here with
> $ make coccicheck M=drivers/gpu/drm/i915/ MODE=context COCCI=./scripts/coccinelle/api/debugfs/debugfs_simple_attr.cocci

Hello Rodrigo,
Thank you so much for your review and feedback on the patch proposal.

>
> The part reported by the this script is the s/SIMPLE/DEBUGFS
> but the change to the unsafe option is not.

If you look at the original commit of this coccinelle file, it calls out the
need for pairing debugfs_create_file_unsafe() as well. Please review this

commitID: 5103068eaca2: ("debugfs, coccinelle: check for obsolete DEFINE_SIMPLE_ATTRIBUTE() usage")

Based on my review of the code, the functions debugfs_create_file() and
debugfs_create_file_unsafe(), both internally call __debugfs_create_file().
However, they pass debugfs_full_proxy_file_operations and
debugfs_open_proxy_file_operations respectively to it. The former represents the
full proxy factory, where as the later one is lightweight open proxy
implementation of the file operations structure.

>
> This commit message is not explaining why the unsafe is the suggested
> or who suggested it.

If you find the response above accurate, I will include these details about
the _unsafe() function in my commit message in v2.

>
> If you remove the unsafe part feel free to resend adding:

Please confirm you still believe switching to _unsafe() is not necessary.

>
> Reviewed-by: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> (to both patches, this and the drrs one.
>
> Also, it looks like you could contribute with other 2 patches:
> drivers/gpu/drm/i915/pxp/intel_pxp_debugfs.c:64:0-23: WARNING: pxp_terminate_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
> drivers/gpu/drm/i915/gvt/debugfs.c:150:0-23: WARNING: vgpu_scan_nonprivbb_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE

Yes, these are on my list. Was waiting for a feedback on the first submission
before I send more similar patches.

Appreciate your time and the feedback.


Regards,
./drv

>
> >
> > Signed-off-by: Deepak R Varma <drv@xxxxxxxxx>
> > ---
> >  drivers/gpu/drm/i915/display/intel_fbc.c | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_fbc.c b/drivers/gpu/drm/i915/display/intel_fbc.c
> > index b5ee5ea0d010..4b481e2f908b 100644
> > --- a/drivers/gpu/drm/i915/display/intel_fbc.c
> > +++ b/drivers/gpu/drm/i915/display/intel_fbc.c
> > @@ -1809,10 +1809,10 @@ static int intel_fbc_debugfs_false_color_set(void *data, u64 val)
> >  	return 0;
> >  }
> >
> > -DEFINE_SIMPLE_ATTRIBUTE(intel_fbc_debugfs_false_color_fops,
> > -			intel_fbc_debugfs_false_color_get,
> > -			intel_fbc_debugfs_false_color_set,
> > -			"%llu\n");
> > +DEFINE_DEBUGFS_ATTRIBUTE(intel_fbc_debugfs_false_color_fops,
> > +			 intel_fbc_debugfs_false_color_get,
> > +			 intel_fbc_debugfs_false_color_set,
> > +			 "%llu\n");
> >
> >  static void intel_fbc_debugfs_add(struct intel_fbc *fbc,
> >  				  struct dentry *parent)
> > @@ -1821,8 +1821,8 @@ static void intel_fbc_debugfs_add(struct intel_fbc *fbc,
> >  			    fbc, &intel_fbc_debugfs_status_fops);
> >
> >  	if (fbc->funcs->set_false_color)
> > -		debugfs_create_file("i915_fbc_false_color", 0644, parent,
> > -				    fbc, &intel_fbc_debugfs_false_color_fops);
> > +		debugfs_create_file_unsafe("i915_fbc_false_color", 0644, parent,
> > +					   fbc, &intel_fbc_debugfs_false_color_fops);
> >  }
> >
> >  void intel_fbc_crtc_debugfs_add(struct intel_crtc *crtc)
> > --
> > 2.34.1
> >
> >
> >