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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     Hector Martin <marcan@marcan.st>,
        Stefan Richter <stefanr@s5r6.in-berlin.de>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.15 001/189] firewire-ohci: work around oversized
 DMA reads on JMicron controllers
Thread-Topic: [PATCH AUTOSEL for 4.15 001/189] firewire-ohci: work around
 oversized DMA reads on JMicron controllers
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Date:   Mon, 9 Apr 2018 00:16:44 +0000
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From: Hector Martin <marcan@marcan.st>

[ Upstream commit 188775181bc05f29372b305ef96485840e351fde ]

At least some JMicron controllers issue buggy oversized DMA reads when
fetching context descriptors, always fetching 0x20 bytes at once for
descriptors which are only 0x10 bytes long. This is often harmless, but
can cause page faults on modern systems with IOMMUs:

DMAR: [DMA Read] Request device [05:00.0] fault addr fff56000 [fault reason=
 06] PTE Read access is not set
firewire_ohci 0000:05:00.0: DMA context IT0 has stopped, error code: evt_de=
scriptor_read

This works around the problem by always leaving 0x10 padding bytes at
the end of descriptor buffer pages, which should be harmless to do
unconditionally for controllers in case others have the same behavior.

Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 drivers/firewire/ohci.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/firewire/ohci.c b/drivers/firewire/ohci.c
index ccf52368a073..45c048751f3b 100644
--- a/drivers/firewire/ohci.c
+++ b/drivers/firewire/ohci.c
@@ -1128,7 +1128,13 @@ static int context_add_buffer(struct context *ctx)
 		return -ENOMEM;
=20
 	offset =3D (void *)&desc->buffer - (void *)desc;
-	desc->buffer_size =3D PAGE_SIZE - offset;
+	/*
+	 * Some controllers, like JMicron ones, always issue 0x20-byte DMA reads
+	 * for descriptors, even 0x10-byte ones. This can cause page faults when
+	 * an IOMMU is in use and the oversized read crosses a page boundary.
+	 * Work around this by always leaving at least 0x10 bytes of padding.
+	 */
+	desc->buffer_size =3D PAGE_SIZE - offset - 0x10;
 	desc->buffer_bus =3D bus_addr + offset;
 	desc->used =3D 0;
=20
--=20
2.15.1