drm/amdgpu: Fixes to the AMDGPU EEPROM driver

* When reading from the EEPROM device, there is no
  device limitation on the number of bytes
  read--they're simply sequenced out. Thus, read
  the whole data requested in one go.

* When writing to the EEPROM device, there is a
  256-byte page limit to write to before having to
  generate a STOP on the bus, as well as the
  address written to mustn't cross over the page
  boundary (it actually rolls over). Maximize the
  data written to per bus acquisition.

* Return the number of bytes read/written, or -errno.

* Add kernel doc.

Cc: Jean Delvare <jdelvare@suse.de>
Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Cc: Lijo Lazar <Lijo.Lazar@amd.com>
Cc: Stanley Yang <Stanley.Yang@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_eeprom.c

@@ -24,59 +24,99 @@
 #include "amdgpu_eeprom.h"
 #include "amdgpu.h"
 
-#define EEPROM_OFFSET_LENGTH 2
+/* AT24CM02 has a 256-byte write page size.
+ */
+#define EEPROM_PAGE_BITS   8
+#define EEPROM_PAGE_SIZE   (1U << EEPROM_PAGE_BITS)
+#define EEPROM_PAGE_MASK   (EEPROM_PAGE_SIZE - 1)
+
+#define EEPROM_OFFSET_SIZE 2
 
+/**
+ * amdgpu_eeprom_xfer -- Read/write from/to an I2C EEPROM device
+ * @i2c_adap: pointer to the I2C adapter to use
+ * @slave_addr: I2C address of the slave device
+ * @eeprom_addr: EEPROM address from which to read/write
+ * @eeprom_buf: pointer to data buffer to read into/write from
+ * @buf_size: the size of @eeprom_buf
+ * @read: True if reading from the EEPROM, false if writing
+ *
+ * Returns the number of bytes read/written; -errno on error.
+ */
 int amdgpu_eeprom_xfer(struct i2c_adapter *i2c_adap,
 		       u16 slave_addr, u16 eeprom_addr,
-		       u8 *eeprom_buf, u16 bytes, bool read)
+		       u8 *eeprom_buf, u16 buf_size, bool read)
 {
-	u8 eeprom_offset_buf[2];
-	u16 bytes_transferred;
+	u8 eeprom_offset_buf[EEPROM_OFFSET_SIZE];
 	struct i2c_msg msgs[] = {
 		{
 			.addr = slave_addr,
 			.flags = 0,
-			.len = EEPROM_OFFSET_LENGTH,
+			.len = EEPROM_OFFSET_SIZE,
 			.buf = eeprom_offset_buf,
 		},
 		{
 			.addr = slave_addr,
 			.flags = read ? I2C_M_RD : 0,
-			.len = bytes,
-			.buf = eeprom_buf,
 		},
 	};
+	const u8 *p = eeprom_buf;
 	int r;
+	u16 len;
+
+	r = 0;
+	for (len = 0; buf_size > 0;
+	     buf_size -= len, eeprom_addr += len, eeprom_buf += len) {
+		/* Set the EEPROM address we want to write to/read from.
+		 */
+		msgs[0].buf[0] = (eeprom_addr >> 8) & 0xff;
+		msgs[0].buf[1] = eeprom_addr & 0xff;
 
-	msgs[0].buf[0] = ((eeprom_addr >> 8) & 0xff);
-	msgs[0].buf[1] = (eeprom_addr & 0xff);
+		if (!read) {
+			/* Write the maximum amount of data, without
+			 * crossing the device's page boundary, as per
+			 * its spec. Partial page writes are allowed,
+			 * starting at any location within the page,
+			 * so long as the page boundary isn't crossed
+			 * over (actually the page pointer rolls
+			 * over).
+			 *
+			 * As per the AT24CM02 EEPROM spec, after
+			 * writing into a page, the I2C driver MUST
+			 * terminate the transfer, i.e. in
+			 * "i2c_transfer()" below, with a STOP
+			 * condition, so that the self-timed write
+			 * cycle begins. This is implied for the
+			 * "i2c_transfer()" abstraction.
+			 */
+			len = min(EEPROM_PAGE_SIZE - (eeprom_addr &
+						      EEPROM_PAGE_MASK),
+				  (u32)buf_size);
+		} else {
+			/* Reading from the EEPROM has no limitation
+			 * on the number of bytes read from the EEPROM
+			 * device--they are simply sequenced out.
+			 */
+			len = buf_size;
+		}
+		msgs[1].len = len;
+		msgs[1].buf = eeprom_buf;
 
-	while (msgs[1].len) {
 		r = i2c_transfer(i2c_adap, msgs, ARRAY_SIZE(msgs));
-		if (r <= 0)
-			return r;
+		if (r < ARRAY_SIZE(msgs))
+			break;
 
-		/* Only for write data */
-		if (!msgs[1].flags)
-			/*
-			 * According to EEPROM spec there is a MAX of 10 ms required for
-			 * EEPROM to flush internal RX buffer after STOP was issued at the
-			 * end of write transaction. During this time the EEPROM will not be
-			 * responsive to any more commands - so wait a bit more.
+		if (!read) {
+			/* According to the AT24CM02 EEPROM spec the
+			 * length of the self-writing cycle, tWR, is
+			 * 10 ms.
 			 *
 			 * TODO Improve to wait for first ACK for slave address after
 			 * internal write cycle done.
 			 */
 			msleep(10);
-
-
-		bytes_transferred = r - EEPROM_OFFSET_LENGTH;
-		eeprom_addr += bytes_transferred;
-		msgs[0].buf[0] = ((eeprom_addr >> 8) & 0xff);
-		msgs[0].buf[1] = (eeprom_addr & 0xff);
-		msgs[1].buf += bytes_transferred;
-		msgs[1].len -= bytes_transferred;
+		}
 	}
 
-	return 0;
+	return r < 0 ? r : eeprom_buf - p;
 }