From 3ab73b453016d91b9f942a7c12173854135530a4 Mon Sep 17 00:00:00 2001
From: Peter Hoyes <Peter.Hoyes@arm.com>
Date: Wed, 23 Aug 2023 21:17:17 +0100
Subject: [PATCH] virtio: rng: Workaround for FVP returning zero-size buffer

The FVP virtio-rng device is observed to always 8 fewer bytes of random
data than requested. When 8 of fewer bytes are requested, the FVP
returns 0 bytes. This causes U-Boot to hang upon attempting to fill the
last 8 bytes of the input buffer.

The virtio spec states than entropy devices must always return at least
1 byte of random data.

To workaround this, always request exactly 16 bytes from the virtio
device, discarding any unused data.

Upstream-Status: Inappropriate [Temporary workaround]
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
---
 drivers/virtio/virtio_rng.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/virtio/virtio_rng.c b/drivers/virtio/virtio_rng.c
index b85545c2ee5..1e4fc342406 100644
--- a/drivers/virtio/virtio_rng.c
+++ b/drivers/virtio/virtio_rng.c
@@ -29,7 +29,7 @@ static int virtio_rng_read(struct udevice *dev, void *data, size_t len)

 	while (len) {
 		sg.addr = buf;
-		sg.length = min(len, sizeof(buf));
+		sg.length = sizeof(buf);
 		sgs[0] = &sg;

 		ret = virtqueue_add(priv->rng_vq, sgs, 0, 1);
@@ -44,8 +44,8 @@ static int virtio_rng_read(struct udevice *dev, void *data, size_t len)
 		if (rsize > sg.length)
 			return -EIO;

-		memcpy(ptr, buf, rsize);
-		len -= rsize;
+		memcpy(ptr, buf, min(len, (size_t)rsize));
+		len -= min(len, (size_t)rsize);
 		ptr += rsize;
 	}

--
2.34.1