linux-kernel - [PATCH 6/9] Add comment about why 32-bit read/modify/write isn't safe.

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Message-ID: <20160901164443.14195.13483.stgit@bhelgaas-glaptop2.roam.corp.google.com>
Date:   Thu, 01 Sep 2016 11:44:43 -0500
From:   Bjorn Helgaas <bhelgaas@...gle.com>
To:     Shawn Lin <shawn.lin@...k-chips.com>
Cc:     devicetree@...r.kernel.org, Wenrui Li <wenrui.li@...k-chips.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Arnd Bergmann <arnd@...db.de>,
        Marc Zyngier <marc.zyngier@....com>, linux-pci@...r.kernel.org,
        Brian Norris <briannorris@...omium.org>,
        linux-kernel@...r.kernel.org,
        Doug Anderson <dianders@...omium.org>,
        linux-rockchip@...ts.infradead.org,
        Rob Herring <robh+dt@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>
Subject: [PATCH 6/9] Add comment about why 32-bit read/modify/write isn't
 safe.


---
 drivers/pci/host/pcie-rockchip.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/pci/host/pcie-rockchip.c b/drivers/pci/host/pcie-rockchip.c
index 17612e5..33bf2e1 100644
--- a/drivers/pci/host/pcie-rockchip.c
+++ b/drivers/pci/host/pcie-rockchip.c
@@ -282,6 +282,11 @@ static int rockchip_pcie_wr_own_conf(struct rockchip_pcie *rockchip,
 
 	mask = ~(((1 << (size * 8)) - 1) << ((where & 0x3) * 8));
 
+	/*
+	 * N.B. This read/modify/write isn't safe in general because it can
+	 * corrupt RW1C bits in adjacent registers.  But the hardware
+	 * doesn't support smaller writes.
+	 */
 	tmp = readl(rockchip->apb_base + PCIE_RC_CONFIG_BASE + offset) & mask;
 	tmp |= val << ((where & 0x3) * 8);
 	writel(tmp, rockchip->apb_base + PCIE_RC_CONFIG_BASE + offset);