linux-kernel - [PATCH] char: xillybus: Allow 64-bit DMA on PCIe interface

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Message-Id: <1438769006-11755-1-git-send-email-eli.billauer@gmail.com>
Date:	Wed,  5 Aug 2015 13:03:26 +0300
From:	Eli Billauer <eli.billauer@...il.com>
To:	arnd@...db.de, gregkh@...uxfoundation.org
Cc:	linux-kernel@...r.kernel.org, Eli Billauer <eli.billauer@...il.com>
Subject: [PATCH] char: xillybus: Allow 64-bit DMA on PCIe interface

Until now, only 32-bit DMA addressing was allowed, following a report on
some old Intel machine that dropped 64-bit PCIe packets, even though
pci_set_dma_mask() was successful with DMA_BIT_MASK(64).

But then came TI's Keystone II chip (ARM Cortex A15 + DSPs), which refuses
32-bit DMA addressing (for good reasons). So 64-bit DMA is allowed as a
fallback option.

Signed-off-by: Eli Billauer <eli.billauer@...il.com>
---
 drivers/char/xillybus/xillybus_pcie.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/char/xillybus/xillybus_pcie.c b/drivers/char/xillybus/xillybus_pcie.c
index d8266bc..9418300 100644
--- a/drivers/char/xillybus/xillybus_pcie.c
+++ b/drivers/char/xillybus/xillybus_pcie.c
@@ -193,14 +193,16 @@ static int xilly_probe(struct pci_dev *pdev,
 	}
 
 	/*
-	 * In theory, an attempt to set the DMA mask to 64 and dma_using_dac=1
-	 * is the right thing. But some unclever PCIe drivers report it's OK
-	 * when the hardware drops those 64-bit PCIe packets. So trust
-	 * nobody and use 32 bits DMA addressing in any case.
+	 * Some (old and buggy?) hardware drops 64-bit addressed PCIe packets,
+	 * even when the PCIe driver claims that a 64-bit mask is OK. On the
+	 * other hand, on some architectures, 64-bit addressing is mandatory.
+	 * So go for the 64-bit mask only when failing is the other option.
 	 */
 
 	if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
 		endpoint->dma_using_dac = 0;
+	} else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
+		endpoint->dma_using_dac = 1;
 	} else {
 		dev_err(endpoint->dev, "Failed to set DMA mask. Aborting.\n");
 		return -ENODEV;
-- 
1.7.2.3

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