rapidio/tsi721: add PCIe MRRS override parameter

Add PCIe Maximum Read Request Size (MRRS) adjustment parameter to allow
users to override configuration register value set during PCIe bus
initialization.

Performance of Tsi721 device as PCIe bus master can be improved if MRRS
is set to its maximum value (4096 bytes).  Some platforms have
limitations for supported MRRS and therefore the default value should be
preserved, unless it is known that given platform supports full set of
MRRS values defined by PCI Express specification.

Link: http://lkml.kernel.org/r/1469125134-16523-6-git-send-email-alexandre.bounine@idt.com
Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Andre van Herk <andre.van.herk@prodrive-technologies.com>
Cc: Barry Wood <barry.wood@idt.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/rapidio/tsi721.txt

@@ -39,6 +39,13 @@ fully compatible with RIONET driver (Ethernet over RapidIO messaging services).
         DMA channels not selected by this mask will not be used by this device
         driver. Default value is 0x7f (use all channels).
 
+- 'pcie_mrrs' - override value for PCIe Maximum Read Request Size (MRRS).
+        This parameter gives an ability to override MRRS value set during PCIe
+        configuration process. Tsi721 supports read request sizes up to 4096B.
+        Value for this parameter must be set as defined by PCIe specification:
+        0 = 128B, 1 = 256B, 2 = 512B, 3 = 1024B, 4 = 2048B and 5 = 4096B.
+        Default value is '-1' (= keep platform setting).
+
 II. Known problems
 
   None.

drivers/rapidio/devices/tsi721.c

@@ -37,11 +37,15 @@
 #include "tsi721.h"
 
 #ifdef DEBUG
-u32 dbg_level = DBG_INIT | DBG_EXIT;
+u32 dbg_level;
 module_param(dbg_level, uint, S_IWUSR | S_IRUGO);
 MODULE_PARM_DESC(dbg_level, "Debugging output level (default 0 = none)");
 #endif
 
+static int pcie_mrrs = -1;
+module_param(pcie_mrrs, int, S_IRUGO);
+MODULE_PARM_DESC(pcie_mrrs, "PCIe MRRS override value (0...5)");
+
 static void tsi721_omsg_handler(struct tsi721_device *priv, int ch);
 static void tsi721_imsg_handler(struct tsi721_device *priv, int ch);
 
@@ -2840,6 +2844,16 @@ static int tsi721_probe(struct pci_dev *pdev,
 	pcie_capability_clear_and_set_word(pdev, PCI_EXP_DEVCTL,
 		PCI_EXP_DEVCTL_RELAX_EN | PCI_EXP_DEVCTL_NOSNOOP_EN, 0);
 
+	/* Override PCIe Maximum Read Request Size setting if requested */
+	if (pcie_mrrs >= 0) {
+		if (pcie_mrrs <= 5)
+			pcie_capability_clear_and_set_word(pdev, PCI_EXP_DEVCTL,
+					PCI_EXP_DEVCTL_READRQ, pcie_mrrs << 12);
+		else
+			tsi_info(&pdev->dev,
+				 "Invalid MRRS override value %d", pcie_mrrs);
+	}
+
 	/* Adjust PCIe completion timeout. */
 	pcie_capability_clear_and_set_word(pdev, PCI_EXP_DEVCTL2, 0xf, 0x2);