x86/pci-dma: remove the explicit nodac and allowdac option

This is something drivers should decide (modulo chipset quirks like
for VIA), which as far as I can tell is how things have been handled
for the last 15 years.

Note that we keep the usedac option for now, as it is used in the wild
to override the too generic VIA quirk.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/x86/x86_64/boot-options.txt

@@ -236,11 +236,6 @@ IOMMU (input/output memory management unit)
     nomerge            Don't do scatter-gather (SG) merging.
     noaperture         Ask the IOMMU not to touch the aperture for AGP.
     noagp              Don't initialize the AGP driver and use full aperture.
-    allowdac           Allow double-address cycle (DAC) mode, i.e. DMA >4GB.
-                       DAC is used with 32-bit PCI to push a 64-bit address in
-                       two cycles. When off all DMA over >4GB is forced through
-                       an IOMMU or software bounce buffering.
-    nodac              Forbid DAC mode, i.e. DMA >4GB.
     panic              Always panic when IOMMU overflows.
     calgary            Use the Calgary IOMMU if it is available
 

arch/x86/kernel/pci-dma.c

@@ -122,9 +122,9 @@ static __init int iommu_setup(char *p)
 		if (!strncmp(p, "forcesac", 8))
 			pr_warn("forcesac option ignored.\n");
 		if (!strncmp(p, "allowdac", 8))
-			forbid_dac = 0;
+			pr_warn("allowdac option ignored.\n");
 		if (!strncmp(p, "nodac", 5))
-			forbid_dac = 1;
+			pr_warn("nodac option ignored.\n");
 		if (!strncmp(p, "usedac", 6)) {
 			forbid_dac = -1;
 			return 1;