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Subject[PATCH 8/9] 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 | 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 c3d8c70..41f9ae2 100644
--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -256,12 +256,40 @@ 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);
--
1.7.7.3


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