net: ethernet: pcnet32: Setup the SRAM and NOUFLO on Am79C97{3, 5}

On a MIPS Malta board, tons of fifo underflow errors have been observed
when using u-boot as bootloader instead of YAMON. The reason for that
is that YAMON used to set the pcnet device to SRAM mode but u-boot does
not. As a result, the default Tx threshold (64 bytes) is now too small to
keep the fifo relatively used and it can result to Tx fifo underflow errors.
As a result of which, it's best to setup the SRAM on supported controllers
so we can always use the NOUFLO bit.

Cc: <netdev@vger.kernel.org>
Cc: <stable@vger.kernel.org>
Cc: <linux-kernel@vger.kernel.org>
Cc: Don Fry <pcnet32@frontier.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/amd/pcnet32.c

@@ -1543,7 +1543,7 @@ pcnet32_probe1(unsigned long ioaddr, int shared, struct pci_dev *pdev)
 {
  struct pcnet32_private *lp;
  int i, media;
- int fdx, mii, fset, dxsuflo;
+ int fdx, mii, fset, dxsuflo, sram;
  int chip_version;
  char *chipname;
  struct net_device *dev;
@@ -1580,7 +1580,7 @@ pcnet32_probe1(unsigned long ioaddr, int shared, struct pci_dev *pdev)
  }
 
  /* initialize variables */
- fdx = mii = fset = dxsuflo = 0;
+ fdx = mii = fset = dxsuflo = sram = 0;
  chip_version = (chip_version >> 12) & 0xffff;
 
  switch (chip_version) {
@@ -1613,6 +1613,7 @@ pcnet32_probe1(unsigned long ioaddr, int shared, struct pci_dev *pdev)
  chipname = "PCnet/FAST III 79C973"; /* PCI */
  fdx = 1;
  mii = 1;
+ sram = 1;
  break;
  case 0x2626:
  chipname = "PCnet/Home 79C978"; /* PCI */
@@ -1636,6 +1637,7 @@ pcnet32_probe1(unsigned long ioaddr, int shared, struct pci_dev *pdev)
  chipname = "PCnet/FAST III 79C975"; /* PCI */
  fdx = 1;
  mii = 1;
+ sram = 1;
  break;
  case 0x2628:
  chipname = "PCnet/PRO 79C976";
@@ -1664,6 +1666,31 @@ pcnet32_probe1(unsigned long ioaddr, int shared, struct pci_dev *pdev)
  dxsuflo = 1;
  }
 
+ /*
+ * The Am79C973/Am79C975 controllers come with 12K of SRAM
+ * which we can use for the Tx/Rx buffers but most importantly,
+ * the use of SRAM allow us to use the BCR18:NOUFLO bit to avoid
+ * Tx fifo underflows.
+ */
+ if (sram) {
+ /*
+ * The SRAM is being configured in two steps. First we
+ * set the SRAM size in the BCR25:SRAM_SIZE bits. According
+ * to the datasheet, each bit corresponds to a 512-byte
+ * page so we can have at most 24 pages. The SRAM_SIZE
+ * holds the value of the upper 8 bits of the 16-bit SRAM size.
+ * The low 8-bits start at 0x00 and end at 0xff. So the
+ * address range is from 0x0000 up to 0x17ff. Therefore,
+ * the SRAM_SIZE is set to 0x17. The next step is to set
+ * the BCR26:SRAM_BND midway through so the Tx and Rx
+ * buffers can share the SRAM equally.
+ */
+ a->write_bcr(ioaddr, 25, 0x17);
+ a->write_bcr(ioaddr, 26, 0xc);
+ /* And finally enable the NOUFLO bit */
+ a->write_bcr(ioaddr, 18, a->read_bcr(ioaddr, 18) | (1 << 11));
+ }
+
  dev = alloc_etherdev(sizeof(*lp));
  if (!dev) {
  ret = -ENOMEM;