linux-kernel - [PATCH 3.16 093/366] drm/radeon: Fix PCIe lane width calculation

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Message-ID: <lsq.1539530741.318772711@decadent.org.uk>
Date:   Sun, 14 Oct 2018 16:25:41 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Christian König" <christian.koenig@....com>,
        "Chunming Zhou" <david1.zhou@....com>,
        "Paul Parsons" <lost.distance@...oo.com>,
        "Alex Deucher" <alexander.deucher@....com>
Subject: [PATCH 3.16 093/366] drm/radeon: Fix PCIe lane width calculation

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Paul Parsons <lost.distance@...oo.com>

commit 85e290d92b4b794d0c758c53007eb4248d385386 upstream.

Two years ago I tried an AMD Radeon E8860 embedded GPU with the drm driver.
The dmesg output included driver warnings about an invalid PCIe lane width.
Tracking the problem back led to si_set_pcie_lane_width_in_smc().
The calculation of the lane widths via ATOM_PPLIB_PCIE_LINK_WIDTH_MASK and
ATOM_PPLIB_PCIE_LINK_WIDTH_SHIFT macros did not increment the resulting
value, per the comment in pptable.h ("lanes - 1"), and per usage elsewhere.
Applying the increment silenced the warnings.
The code has not changed since, so either my analysis was incorrect or the
bug has gone unnoticed. Hence submitting this as an RFC.

Acked-by: Christian König <christian.koenig@....com>
Acked-by: Chunming Zhou <david1.zhou@....com>
Signed-off-by: Paul Parsons <lost.distance@...oo.com>
Signed-off-by: Alex Deucher <alexander.deucher@....com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/gpu/drm/radeon/si_dpm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/gpu/drm/radeon/si_dpm.c
+++ b/drivers/gpu/drm/radeon/si_dpm.c
@@ -5828,9 +5828,9 @@ static void si_set_pcie_lane_width_in_sm
 {
 	u32 lane_width;
 	u32 new_lane_width =
-		(radeon_new_state->caps & ATOM_PPLIB_PCIE_LINK_WIDTH_MASK) >> ATOM_PPLIB_PCIE_LINK_WIDTH_SHIFT;
+		((radeon_new_state->caps & ATOM_PPLIB_PCIE_LINK_WIDTH_MASK) >> ATOM_PPLIB_PCIE_LINK_WIDTH_SHIFT) + 1;
 	u32 current_lane_width =
-		(radeon_current_state->caps & ATOM_PPLIB_PCIE_LINK_WIDTH_MASK) >> ATOM_PPLIB_PCIE_LINK_WIDTH_SHIFT;
+		((radeon_current_state->caps & ATOM_PPLIB_PCIE_LINK_WIDTH_MASK) >> ATOM_PPLIB_PCIE_LINK_WIDTH_SHIFT) + 1;
 
 	if (new_lane_width != current_lane_width) {
 		radeon_set_pcie_lanes(rdev, new_lane_width);