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 | 32 ++++++++++++++++++++++++++++++-- 1 files changed, 30 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c index b79a181..61fe317 100644 --- a/drivers/gpu/drm/i915/intel_i2c.c +++ b/drivers/gpu/drm/i915/intel_i2c.c @@ -255,12 +255,40 @@ gmbus_xfer(struct i2c_adapter *adapter, I915_WRITE(GMBUS0 + reg_offset, bus->reg0);
for (i = 0; i < num; i++) { - u16 len = msgs[i].len; - u8 *buf = msgs[i].buf; + u16 len; + u8 *buf; + u32 gmbus5 = 0; + u32 gmbus1_index = 0; + + /* + * The gmbus controller can combine a 1 or 2 byte write with a + * read that immediately follows it by using an "INDEX" cycle. + */ + if (i + 1 < num && + !(msgs[i].flags & I2C_M_RD) && + (msgs[i + 1].flags & I2C_M_RD) && + msgs[i].len <= 2) { + if (msgs[i].len == 2) + gmbus5 = GMBUS_2BYTE_INDEX_EN | + msgs[i].buf[1] | + (msgs[i].buf[0] << 8); + if (msgs[i].len == 1) + gmbus1_index = GMBUS_CYCLE_INDEX | + (msgs[i].buf[0] << + GMBUS_SLAVE_INDEX_SHIFT); + i += 1; /* set i to the index of the read xfer */ + } + + len = msgs[i].len; + buf = msgs[i].buf; + + /* GMBUS5 holds 16-bit index, but must be 0 if not used */ + I915_WRITE(GMBUS5 + reg_offset, gmbus5);
if (msgs[i].flags & I2C_M_RD) { I915_WRITE(GMBUS1 + reg_offset, GMBUS_CYCLE_WAIT | + gmbus1_index | (len << GMBUS_BYTE_COUNT_SHIFT) | (msgs[i].addr << GMBUS_SLAVE_ADDR_SHIFT) | GMBUS_SLAVE_READ | GMBUS_SW_RDY);