[PATCH v2] drm/xe: Make p2pdma distance check verbose via dynamic debug

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

pci_p2pdma_distance() can optionally print details about how it
computed the ACS/PCIe topology distance between the two devices, but
xe_dma_buf_attach() always called it with verbose set to false,
silently hiding useful diagnostics whenever a dma-buf attach unset
attach->peer2peer.

Tie the verbose argument to a dedicated dynamic-debug callsite tagged
"xe dma-buf p2p debug" (in addition to keeping the existing
DRM_UT_DRIVER category as a default), so the extra p2pdma distance
logging can be switched on without a recompile in any of the following
ways:

 - Enable the whole DRIVER category for xe via drm.debug, which is
   already used elsewhere in the driver:

     # echo 0x10 > /sys/module/drm/parameters/debug

 - Enable dynamic debug for the whole file:

     # echo 'file xe_dma_buf.c +p' > \
             /sys/kernel/debug/dynamic_debug/control

 - Enable dynamic debug for just this callsite by function name:

     # echo 'func xe_dma_buf_attach +p' > \
             /sys/kernel/debug/dynamic_debug/control

 - Enable dynamic debug by matching the format string used to
   register the callsite:

     # echo 'format "xe dma-buf p2p debug" +p' > \
             /sys/kernel/debug/dynamic_debug/control

 - Enable it at boot via the dyndbg= kernel command line parameter:

     dyndbg="func xe_dma_buf_attach +p"

Any of the above flips verbose_p2p_dist to true for that attach call,
causing pci_p2pdma_distance() to log its distance calculation without
requiring the whole DRM_UT_DRIVER category to be enabled.

v2:
- Guard the _dynamic_func_call() invocation and
  xe_ddebug_flip_bool() with a CONFIG_DYNAMIC_DEBUG check so the
  build does not break when dynamic debug is disabled. (Sashiko-bot)

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Assisted-by: LLM
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
---
 drivers/gpu/drm/xe/xe_dma_buf.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/xe/xe_dma_buf.c b/drivers/gpu/drm/xe/xe_dma_buf.c
index 5d9f1cd24b7f..6a85b292dee7 100644
--- a/drivers/gpu/drm/xe/xe_dma_buf.c
+++ b/drivers/gpu/drm/xe/xe_dma_buf.c
@@ -7,6 +7,7 @@
 
 #include <kunit/test.h>
 #include <linux/dma-buf.h>
+#include <linux/dynamic_debug.h>
 #include <linux/pci-p2pdma.h>
 
 #include <drm/drm_device.h>
@@ -22,13 +23,29 @@
 
 MODULE_IMPORT_NS("DMA_BUF");
 
+#if defined(CONFIG_DYNAMIC_DEBUG) || \
+	(defined(CONFIG_DYNAMIC_DEBUG_CORE) && defined(DYNAMIC_DEBUG_MODULE))
+static void xe_ddebug_flip_bool(const struct _ddebug *desc, bool *flag)
+{
+	*flag = true;
+}
+
+#define xe_dma_buf_p2p_debug_flip(flag) \
+	_dynamic_func_call("xe dma-buf p2p debug", xe_ddebug_flip_bool, flag)
+#else
+#define xe_dma_buf_p2p_debug_flip(flag) do { } while (0)
+#endif
+
 static int xe_dma_buf_attach(struct dma_buf *dmabuf,
 			     struct dma_buf_attachment *attach)
 {
 	struct drm_gem_object *obj = attach->dmabuf->priv;
+	bool verbose_p2p_dist = drm_debug_enabled(DRM_UT_DRIVER);
+
+	xe_dma_buf_p2p_debug_flip(&verbose_p2p_dist);
 
 	if (attach->peer2peer &&
-	    pci_p2pdma_distance(to_pci_dev(obj->dev->dev), attach->dev, false) < 0)
+	    pci_p2pdma_distance(to_pci_dev(obj->dev->dev), attach->dev, verbose_p2p_dist) < 0)
 		attach->peer2peer = false;
 
 	if (!attach->peer2peer && !xe_bo_can_migrate(gem_to_xe_bo(obj), XE_PL_TT))
-- 
2.55.0