Re: [Intel-gfx] [PATCH 1/1] drm/i915/dsi: silence a warning about uninitialized return value

[Dave Gordon]

On 06/09/16 21:36, Nicolas Iooss wrote:
> On 06/09/16 12:21, Dave Gordon wrote:
> > On 04/09/16 19:58, Nicolas Iooss wrote:
> > > When building the kernel with clang and some warning flags, the compiler
> > > reports that the return value of dcs_get_backlight() may be
> > > uninitialized:
> > >
> > >     drivers/gpu/drm/i915/intel_dsi_dcs_backlight.c:53:2: error: variable
> > >     'data' is used uninitialized whenever 'for' loop exits because its
> > >     condition is false [-Werror,-Wsometimes-uninitialized]
> > >             for_each_dsi_port(port, intel_dsi->dcs_backlight_ports) {
> > >             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > >     drivers/gpu/drm/i915/intel_dsi.h:126:49: note: expanded from macro
> > >     'for_each_dsi_port'
> > >     #define for_each_dsi_port(__port, __ports_mask)
> > >                                 for_each_port_masked(__port,
> > > __ports_mask)
> > >
> > > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > >     drivers/gpu/drm/i915/i915_drv.h:322:26: note: expanded from macro
> > >     'for_each_port_masked'
> > >         for ((__port) = PORT_A; (__port) < I915_MAX_PORTS; (__port)++)  \
> > >                                 ^~~~~~~~~~~~~~~~~~~~~~~~~
> > >     drivers/gpu/drm/i915/intel_dsi_dcs_backlight.c:60:9: note:
> > >     uninitialized use occurs here
> > >             return data;
> > >                    ^~~~
> > >
> > > As intel_dsi->dcs_backlight_ports seems to be always initialized to a
> > > non-null value, the content of the for loop is always executed and there
> > > is no bug in the current code. Nevertheless the compiler has no way of
> > > knowing that assumption, so initialize variable 'data' to silence the
> > > warning here.
> > >
> > > Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@xxxxxxx>
> >
> > Interesting ... there are two things that could lead to this (possibly)
> > incorrect analysis. Either it thinks the loop could be executed zero
> > times, which would be a deficiency in the compiler, as the initialiser
> > and loop bound are both known (different) constants:
> >
> > enum port {
> >         PORT_A = 0,
> >         PORT_B,
> >         PORT_C,
> >         PORT_D,
> >         PORT_E,
> >         I915_MAX_PORTS
> > };
> >
> > or, it doesn't understand that because we've passed &data to another
> > function, it can have been set by the callee. It may be extra confusing
> > that the callee takes (void *); or it may be being ultra-sophisticated
> > in its analysis and noted that in one error path data is *not* set (and
> > we then discard the error and use data anyway). As an experiment, you
> > could try:
>
> The code that the compiler sees is not a simple loop other enum 'port'
> but "for_each_dsi_port(port, intel_dsi->dcs_backlight_ports) {", which
> is expanded [1] to:
>
>     for ((port) = PORT_A; (port) < I915_MAX_PORTS; (port)++)
>       if (!((intel_dsi->dcs_backlight_ports) & (1 << (port)))) {} else {
>
> This is why I spoke of intel_dsi->dcs_backlight_ports in my description:
> if it is zero, the body of the loop is never run.
>
> As for the analyses of calls using &data, clang does not warn about the
> variable being maybe uninitialized following a call. This is quite
> expected as this would lead to too many false positives, even though it
> may miss some bugs.
>
> > static u8 mipi_dsi_dcs_read1(struct mipi_dsi_device *dsi_device, u8 cmd)
> > {
> >         u8 data = 0;
> >
> >         mipi_dsi_dcs_read(dsi_device, cmd, &data, sizeof(data));
> >
> >         return data;
> > }
> >
> > static u32 dcs_get_backlight(struct intel_connector *connector)
> > {
> >         struct intel_encoder *encoder = connector->encoder;
> >         struct intel_dsi *intel_dsi = enc_to_intel_dsi(&encoder->base);
> >         struct mipi_dsi_device *dsi_device;
> >         enum port port;
> >         u8 data;
> >
> >         /* FIXME: Need to take care of 16 bit brightness level */
> >         for_each_dsi_port(port, intel_dsi->dcs_backlight_ports) {
> >                 dsi_device = intel_dsi->dsi_hosts[port]->device;
> >                 data = mipi_dsi_dcs_read1(dsi_device,
> > MIPI_DCS_GET_DISPLAY_BRIGHTNESS);
> >                 break;
> >         }
> >
> >         return data;
> > }
> >
> > If it complains about that then it's a shortcoming in the loop analysis.
>
> It complains (in dcs_get_backlight), because for_each_dsi_port() still
> hides an 'if' condition.

So it does, In that case the complaint is really quite reasonable.

> > If not you could try:
> >
> > static u8 mipi_dsi_dcs_read1(struct mipi_dsi_device *dsi_device, u8 cmd)
> > {
> >         u8 data;
> >     ssize_t nbytes = sizeof(data);
> >
> >     nbytes = mipi_dsi_dcs_read(dsi_device, cmd, &data, nbytes);
> >     return nbytes == sizeof(data) ? data : 0;
> > }
> >
> > and if complains about that then it doesn't understand that passing
> > &data allows it to be set. If it doesn't complain about this version,
> > then the original error was actually correct, in the sense that data can
> > indeed be used uninitialised if certain error paths can be taken.
>
> clang did not complain with this last case.

It probably should have, since the (hidden) if() could still result in 
this function never being called. Oh well ...

.Dave.

> > Here's an R-b for your fix anyway ...
> >
> > Reviewed-by: Dave Gordon <david.s.gordon@xxxxxxxxx>
>
> Thanks!
> Nicolas
>
> [1] I used "make drivers/gpu/drm/i915/intel_dsi_dcs_backlight.i" to see
> the output of the preprocessor.