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Date:   Fri,  8 Jan 2021 11:41:30 +0800
From:   Colin Xu <colin.xu@...el.com>
To:     virtualization@...ts.linux-foundation.org
Cc:     kraxel@...hat.com, mst@...hat.com, jasowang@...hat.com,
        linux-kernel@...r.kernel.org, colin.xu@...el.com
Subject: [PATCH] virtio_input: Prevent EV_MSC/MSC_TIMESTAMP loop storm for MT.

In 'commit 29cc309d8bf1 ("HID: hid-multitouch: forward MSC_TIMESTAMP")',
EV_MSC/MSC_TIMESTAMP is added to each before EV_SYN event. EV_MSC is
configured as INPUT_PASS_TO_ALL.
In case of a touch device which report MSC_TIMESTAMP:
  BE pass EV_MSC/MSC_TIMESTAMP to FE on receiving event from evdev.
  FE pass EV_MSC/MSC_TIMESTAMP back to BE.
  BE writes EV_MSC/MSC_TIMESTAMP to evdev due to INPUT_PASS_TO_ALL.
  BE receives extra EV_MSC/MSC_TIMESTAMP and pass to FE.
>>> Each new frame becomes larger and larger.

Disable EV_MSC/MSC_TIMESTAMP forwarding for MT.

Signed-off-by: Colin Xu <colin.xu@...el.com>
---
 drivers/virtio/virtio_input.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/drivers/virtio/virtio_input.c b/drivers/virtio/virtio_input.c
index f1f6208edcf5..244965c20d9b 100644
--- a/drivers/virtio/virtio_input.c
+++ b/drivers/virtio/virtio_input.c
@@ -64,6 +64,21 @@ static int virtinput_send_status(struct virtio_input *vi,
 	unsigned long flags;
 	int rc;
 
+	/*
+	 * Since 29cc309d8bf1 (HID: hid-multitouch: forward MSC_TIMESTAMP),
+	 * EV_MSC/MSC_TIMESTAMP is added to each before EV_SYN event.
+	 * EV_MSC is configured as INPUT_PASS_TO_ALL.
+	 * In case of touch device:
+	 *   BE pass EV_MSC/MSC_TIMESTAMP to FE on receiving event from evdev.
+	 *   FE pass EV_MSC/MSC_TIMESTAMP back to BE.
+	 *   BE writes EV_MSC/MSC_TIMESTAMP to evdev due to INPUT_PASS_TO_ALL.
+	 *   BE receives extra EV_MSC/MSC_TIMESTAMP and pass to FE.
+	 *   >>> Each new frame becomes larger and larger.
+	 * Disable EV_MSC/MSC_TIMESTAMP forwarding for MT.
+	 */
+	if (vi->idev->mt && type == EV_MSC && code == MSC_TIMESTAMP)
+		return 0;
+
 	stsbuf = kzalloc(sizeof(*stsbuf), GFP_ATOMIC);
 	if (!stsbuf)
 		return -ENOMEM;
-- 
2.30.0

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