[PATCH v2 2/2] alpha: disable DAC for 32-bit PCI cards on Tsunami/Typhoon

[Magnus Lindholm <linmag7@gmail.com>]

The Tsunami/Typhoon Pchip's DAC ("monster window") path corrupts data
when used by 32-bit PCI cards using DAC addresses above 4 GiB, even on
cards whose DAC support is otherwise solid: the same cards work
correctly with DAC on Rawhide (MCPCIA) systems, and native 64-bit PCI
cards are unaffected on Tsunami/Typhoon itself. Corruption shows up as
64-byte chunks (one 21264 cache block) of unrelated data - typically
identifiable content belonging to other processes' concurrent DMA -
substituted into the transfer; the rate varies from none to several
kilobytes per run and has not been tied to any particular alignment.

Work around this by capping affected devices to 32-bit DMA, which
routes them through the existing scatter-gather window instead of DAC.
Conventional PCI provides no status bit to distinguish a 32-bit from a
64-bit option card, so use the presence of a 64-bit memory BAR as a
practical proxy. This covers every affected card seen so far, but is
a proxy rather than a direct test: it will also needlessly restrict a
handful of 64-bit cards that only expose 32-bit BARs (e.g. QLogic
ISP1080, ISP10160). These controllers are not known to be supported by
SRM firmware and are therefore uncommon on Alpha systems, so the
trade-off is accepted.

The only driver currently known to hit this is qla1280 with an
ISP1040 card and a 64-bit DMA mask, which is a common and
SRM-supported configuration on Alpha.

Suggested-by: Maciej Rozycki <macro@orcam.me.uk>
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/kernel/pci.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/alpha/kernel/pci.c b/arch/alpha/kernel/pci.c
index 11df411b1d18..7dbf380acd93 100644
--- a/arch/alpha/kernel/pci.c
+++ b/arch/alpha/kernel/pci.c
@@ -23,6 +23,8 @@
 #include <linux/cache.h>
 #include <linux/slab.h>
 #include <linux/syscalls.h>
+#include <linux/dma-mapping.h>
+#include <asm/hwrpb.h>
 #include <asm/machvec.h>
 
 #include "proto.h"
@@ -117,6 +119,26 @@ static void pcibios_fixup_final(struct pci_dev *dev)
 }
 DECLARE_PCI_FIXUP_FINAL(PCI_ANY_ID, PCI_ANY_ID, pcibios_fixup_final);
 
+/*
+ * Tsunami/Typhoon's DAC "monster window" corrupts data on 32-bit PCI
+ * cards; cap them to 32-bit DMA by proxy of having no 64-bit BAR.
+ */
+static void tsunami_dac_quirk(struct pci_dev *pdev)
+{
+	int i;
+
+	if (hwrpb->sys_type != ST_DEC_TSUNAMI)
+		return;
+
+	for (i = 0; i <= PCI_STD_RESOURCE_END; i++)
+		if (pci_resource_flags(pdev, i) & IORESOURCE_MEM_64)
+			return;
+
+	pdev->dev.bus_dma_limit = DMA_BIT_MASK(32);
+	dev_dbg(&pdev->dev, "disabling DAC for device\n");
+}
+DECLARE_PCI_FIXUP_FINAL(PCI_ANY_ID, PCI_ANY_ID, tsunami_dac_quirk);
+
 /* Just declaring that the power-of-ten prefixes are actually the
    power-of-two ones doesn't make it true :) */
 #define KB			1024
-- 
2.53.0