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Subject[PATCH 5/7 v6] drm/i915/intel_i2c: use INDEX cycles for i2c read transactions
Date
It is very common for an i2c device to require a small 1 or 2 byte write
followed by a read. For example, when reading from an i2c EEPROM it is
common to write and address, offset or index followed by a reading some
values.

The i915 gmbus controller provides a special "INDEX" cycle for performing
such a small write followed by a read. The INDEX can be either one or two
bytes long. The advantage of using such a cycle is that the CPU has
slightly less work to do once the read with INDEX cycle is started.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
---
drivers/gpu/drm/i915/intel_i2c.c | 54 +++++++++++++++++++++++++++++++++++---
1 files changed, 50 insertions(+), 4 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
index 25f2c9e..9c07ff2 100644
--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -204,13 +204,15 @@ intel_gpio_setup(struct intel_gmbus *bus, u32 pin)
}

static int
-gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg)
+gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
+ u32 gmbus1_index)
{
int reg_offset = dev_priv->gpio_mmio_base;
u16 len = msg->len;
u8 *buf = msg->buf;

I915_WRITE(GMBUS1 + reg_offset,
+ gmbus1_index |
GMBUS_CYCLE_WAIT |
(len << GMBUS_BYTE_COUNT_SHIFT) |
(msg->addr << GMBUS_SLAVE_ADDR_SHIFT) |
@@ -276,6 +278,46 @@ gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg)
return 0;
}

+/*
+ * The gmbus controller can combine a 1 or 2 byte write with a read that
+ * immediately follows it by using an "INDEX" cycle.
+ */
+static bool
+gmbus_is_index_read(struct i2c_msg *msgs, int i, int num)
+{
+ return (i + 1 < num &&
+ !(msgs[i].flags & I2C_M_RD) && msgs[i].len <= 2 &&
+ (msgs[i + 1].flags & I2C_M_RD));
+}
+
+static int
+gmbus_xfer_index_read(struct drm_i915_private *dev_priv, struct i2c_msg *msgs)
+{
+ int reg_offset = dev_priv->gpio_mmio_base;
+ u32 gmbus1_index = 0;
+ u32 gmbus5 = 0;
+ int ret;
+
+ if (msgs[0].len == 2)
+ gmbus5 = GMBUS_2BYTE_INDEX_EN |
+ msgs[0].buf[1] | (msgs[0].buf[0] << 8);
+ if (msgs[0].len == 1)
+ gmbus1_index = GMBUS_CYCLE_INDEX |
+ (msgs[0].buf[0] << GMBUS_SLAVE_INDEX_SHIFT);
+
+ /* GMBUS5 holds 16-bit index */
+ if (gmbus5)
+ I915_WRITE(GMBUS5 + reg_offset, gmbus5);
+
+ ret = gmbus_xfer_read(dev_priv, &msgs[1], gmbus1_index);
+
+ /* Clear GMBUS5 after each index transfer */
+ if (gmbus5)
+ I915_WRITE(GMBUS5 + reg_offset, 0);
+
+ return ret;
+}
+
static int
gmbus_xfer(struct i2c_adapter *adapter,
struct i2c_msg *msgs,
@@ -300,10 +342,14 @@ gmbus_xfer(struct i2c_adapter *adapter,
I915_WRITE(GMBUS0 + reg_offset, bus->reg0);

for (i = 0; i < num; i++) {
- if (msgs[i].flags & I2C_M_RD)
- ret = gmbus_xfer_read(dev_priv, &msgs[i]);
- else
+ if (gmbus_is_index_read(msgs, i, num)) {
+ ret = gmbus_xfer_index_read(dev_priv, &msgs[i]);
+ i += 1; /* set i to the index of the read xfer */
+ } else if (msgs[i].flags & I2C_M_RD) {
+ ret = gmbus_xfer_read(dev_priv, &msgs[i], 0);
+ } else {
ret = gmbus_xfer_write(dev_priv, &msgs[i]);
+ }

if (ret == -ETIMEDOUT)
goto timeout;
--
1.7.7.3


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