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@m4x.org
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.
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.
Here's an R-b for your fix anyway ...
Reviewed-by: Dave Gordon david.s.gordon@intel.com
Thanks!
Nicolas
[1] I used "make drivers/gpu/drm/i915/intel_dsi_dcs_backlight.i" to see the output of the preprocessor.