[PATCH 2/2] drm/edid: report latency due to reading edids

[Daniel Vetter]

A forced EDID read takes 22.5ms best-case, and that's per 128byte
block. HDMI screens tend to have 2-3 of those. Mutliply that by a few
outputs and then it's clear that userspace really never ever should
re-probe connector state on its own and trust the kernel to tell it
when anything changed. The only exception is a manual reprobe button that
the user must press itself (for extremely shitty KVM switches that
don't wire up hotplug handling properly).

There have been bugs in the past, but we're slowly fixing them up. To
the point even that some of the most abused interfaces (e.g. in sysfs)
have been changed to only return the cached state ever due to too much
polling by userspace.

But there's other places where we can't pull these tricks, so give
userspace the tools to notice their abuse and expose delays due to
EDID reads in latencytop.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
---
 drivers/gpu/drm/drm_edid.c | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index c214f1246cb4..370003e0cc69 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -32,6 +32,7 @@
 #include <linux/hdmi.h>
 #include <linux/i2c.h>
 #include <linux/module.h>
+#include <linux/latencytop.h>
 #include <drm/drmP.h>
 #include <drm/drm_edid.h>
 #include <drm/drm_displayid.h>
@@ -1272,14 +1273,17 @@ struct edid *drm_do_get_edid(struct drm_connector *connector,
 	int i, j = 0, valid_extensions = 0;
 	u8 *block, *new;
 	bool print_bad_edid = !connector->bad_edid_counter || (drm_debug & DRM_UT_KMS);
+	u64 before, after;
 
 	if ((block = kmalloc(EDID_LENGTH, GFP_KERNEL)) == NULL)
 		return NULL;
 
+	before = ktime_get_raw_ns();
+
 	/* base block fetch */
 	for (i = 0; i < 4; i++) {
 		if (get_edid_block(data, block, 0, EDID_LENGTH))
-			goto out;
+			goto none;
 		if (drm_edid_block_valid(block, 0, print_bad_edid,
 					 &connector->edid_corrupt))
 			break;
@@ -1293,11 +1297,11 @@ struct edid *drm_do_get_edid(struct drm_connector *connector,
 
 	/* if there's no extensions, we're done */
 	if (block[0x7e] == 0)
-		return (struct edid *)block;
+		goto out;
 
 	new = krealloc(block, (block[0x7e] + 1) * EDID_LENGTH, GFP_KERNEL);
 	if (!new)
-		goto out;
+		goto none;
 	block = new;
 
 	for (j = 1; j <= block[0x7e]; j++) {
@@ -1305,7 +1309,7 @@ struct edid *drm_do_get_edid(struct drm_connector *connector,
 			if (get_edid_block(data,
 				  block + (valid_extensions + 1) * EDID_LENGTH,
 				  j, EDID_LENGTH))
-				goto out;
+				goto none;
 			if (drm_edid_block_valid(block + (valid_extensions + 1)
 						 * EDID_LENGTH, j,
 						 print_bad_edid,
@@ -1329,11 +1333,11 @@ struct edid *drm_do_get_edid(struct drm_connector *connector,
 		block[0x7e] = valid_extensions;
 		new = krealloc(block, (valid_extensions + 1) * EDID_LENGTH, GFP_KERNEL);
 		if (!new)
-			goto out;
+			goto none;
 		block = new;
 	}
 
-	return (struct edid *)block;
+	goto out;
 
 carp:
 	if (print_bad_edid) {
@@ -1342,9 +1346,16 @@ carp:
 	}
 	connector->bad_edid_counter++;
 
-out:
+none:
 	kfree(block);
-	return NULL;
+	block = NULL;
+
+out:
+	after = ktime_get_raw_ns();
+
+	account_latency(DIV_ROUND_UP_ULL(after - before, 1000));
+
+	return (struct edid *)block;
 }
 EXPORT_SYMBOL_GPL(drm_do_get_edid);
 
-- 
2.5.1

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