drm_debug是cmdline参数，可以trace相关日志，阅读gpu驱动代码时，会非常有效。

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unsigned int drm_debug = 0;
EXPORT_SYMBOL(drm_debug);
 
MODULE_AUTHOR("Gareth Hughes, Leif Delgass, Jos?Fonseca, Jon Smirl");
MODULE_DESCRIPTION("DRM shared core routines");
MODULE_LICENSE("GPL and additional rights");
MODULE_PARM_DESC(debug, "Enable debug output, where each bit enables a debug category.\n"
"\t\tBit 0 (0x01)  will enable CORE messages (drm core code)\n"
"\t\tBit 1 (0x02)  will enable DRIVER messages (drm controller code)\n"
"\t\tBit 2 (0x04)  will enable KMS messages (modesetting code)\n"
"\t\tBit 3 (0x08)  will enable PRIME messages (prime code)\n"
"\t\tBit 4 (0x10)  will enable ATOMIC messages (atomic code)\n"
"\t\tBit 5 (0x20)  will enable VBL messages (vblank code)\n"
"\t\tBit 7 (0x80)  will enable LEASE messages (leasing code)\n"
"\t\tBit 8 (0x100) will enable DP messages (displayport code)");
module_param_named(debug, drm_debug, int, 0600);

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//if want to trace DRM_DEBUG_KMS and DRM_DEBUG_DRIVER log
 drm.debug=0x6

https://elixir.bootlin.com/linux/v5.4.73/source/include/drm/drm_print.h#L230

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/*
 * The following categories are defined:
 *
 * CORE: Used in the generic drm code: drm_ioctl.c, drm_mm.c, drm_memory.c, ...
 *	 This is the category used by the DRM_DEBUG() macro.
 *
 * DRIVER: Used in the vendor specific part of the driver: i915, radeon, ...
 *	   This is the category used by the DRM_DEBUG_DRIVER() macro.
 *
 * KMS: used in the modesetting code.
 *	This is the category used by the DRM_DEBUG_KMS() macro.
 *
 * PRIME: used in the prime code.
 *	  This is the category used by the DRM_DEBUG_PRIME() macro.
 *
 * ATOMIC: used in the atomic code.
 *	  This is the category used by the DRM_DEBUG_ATOMIC() macro.
 *
 * VBL: used for verbose debug message in the vblank code
 *	  This is the category used by the DRM_DEBUG_VBL() macro.
 *
 * Enabling verbose debug messages is done through the drm.debug parameter,
 * each category being enabled by a bit.
 *
 * drm.debug=0x1 will enable CORE messages
 * drm.debug=0x2 will enable DRIVER messages
 * drm.debug=0x3 will enable CORE and DRIVER messages
 * ...
 * drm.debug=0x3f will enable all messages
 *
 * An interesting feature is that it's possible to enable verbose logging at
 * run-time by echoing the debug value in its sysfs node:
 *   # echo 0xf > /sys/module/drm/parameters/debug
 */