Re: [PATCH v4 2/2] drm/i915/gmbus: Enable burst read

[Ramalingam C <ramalingam.c@intel.com>]

As per the discussion at #intel-gfx IRC, I have submitted v5 with the 
max burst read length fixed to 767Bytes. Gist of the discussion: As per 
HW spec HW supports burst for all the msg length above 511Bytes. But for 
512 there is a special handling. Reasoning for this special handling is 
not clear. because of this we are not sure whether other msg lengths in 
multiples of 256 also will have problem or not. As of now, In real world 
we couldn't have an I2C slave on DDC Bus whoc an provide msgs of 
 >511Bytes. Except 534Byte HDCP2.2 Cert Msg. So it is not possible to 
test and confirm the HW capability for n*256Bytes where as n is >= 3. so 
until we get further clarity, we are limiting the burst read support to 
the msgs of length 512Bytes to 767Bytes. --Ram


On Wednesday 09 May 2018 07:15 PM, Ramalingam C wrote:
> Support for Burst read in HW is added for HDCP2.2 compliance
> requirement.
>
> This patch enables the burst read for all the gmbus read of more than
> 511Bytes, on capable platforms.
>
> v2:
>    Extra line is removed.
> v3:
>    Macro is added for detecting the BURST_READ Support [Jani]
>    Runtime detection of the need for burst_read [Jani]
>    Calculation enhancement.
> v4:
>    GMBUS0 reg val is passed from caller [ville]
>    Removed a extra var [ville]
>    Extra brackets are removed [ville]
>    Implemented the handling of 512Bytes Burst Read.
>
> Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_drv.h  |  3 +++
>   drivers/gpu/drm/i915/i915_reg.h  |  1 +
>   drivers/gpu/drm/i915/intel_i2c.c | 52 ++++++++++++++++++++++++++++++++--------
>   3 files changed, 46 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 028691108125..14293fc1a142 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2552,6 +2552,9 @@ intel_info(const struct drm_i915_private *dev_priv)
>    */
>   #define HAS_AUX_IRQ(dev_priv)   true
>   #define HAS_GMBUS_IRQ(dev_priv) (INTEL_GEN(dev_priv) >= 4)
> +#define HAS_GMBUS_BURST_READ(dev_priv) (INTEL_GEN(dev_priv) >= 10 || \
> +					IS_GEMINILAKE(dev_priv) || \
> +					IS_KABYLAKE(dev_priv))
>   
>   /* With the 945 and later, Y tiling got adjusted so that it was 32 128-byte
>    * rows, which changed the alignment requirements and fence programming.
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index df998c10c48e..1166a24aff48 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -2996,6 +2996,7 @@ enum i915_power_well_id {
>   #define   GMBUS_RATE_400KHZ	(2<<8) /* reserved on Pineview */
>   #define   GMBUS_RATE_1MHZ	(3<<8) /* reserved on Pineview */
>   #define   GMBUS_HOLD_EXT	(1<<7) /* 300ns hold time, rsvd on Pineview */
> +#define   GMBUS_BYTE_CNT_OVERRIDE (1<<6)
>   #define   GMBUS_PIN_DISABLED	0
>   #define   GMBUS_PIN_SSC		1
>   #define   GMBUS_PIN_VGADDC	2
> diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
> index 1c0f6b56b209..2b59f8db42f2 100644
> --- a/drivers/gpu/drm/i915/intel_i2c.c
> +++ b/drivers/gpu/drm/i915/intel_i2c.c
> @@ -371,12 +371,30 @@ unsigned int gmbus_max_xfer_size(struct drm_i915_private *dev_priv)
>   static int
>   gmbus_xfer_read_chunk(struct drm_i915_private *dev_priv,
>   		      unsigned short addr, u8 *buf, unsigned int len,
> -		      u32 gmbus1_index)
> +		      u32 gmbus0_reg, u32 gmbus1_index)
>   {
> +	unsigned int size = len;
> +	bool burst_read = len > gmbus_max_xfer_size(dev_priv);
> +	bool extra_byte_added = false;
> +
> +	if (burst_read) {
> +
> +		/*
> +		 * As per HW Spec, for 512Bytes need to read extra Byte and
> +		 * Ignore the extra byte read.
> +		 */
> +		if (len == 512) {
> +			extra_byte_added = true;
> +			len++;
> +		}
> +		size = len % 256 + 256;
> +		I915_WRITE_FW(GMBUS0, gmbus0_reg | GMBUS_BYTE_CNT_OVERRIDE);
> +	}
> +
>   	I915_WRITE_FW(GMBUS1,
>   		      gmbus1_index |
>   		      GMBUS_CYCLE_WAIT |
> -		      (len << GMBUS_BYTE_COUNT_SHIFT) |
> +		      (size << GMBUS_BYTE_COUNT_SHIFT) |
>   		      (addr << GMBUS_SLAVE_ADDR_SHIFT) |
>   		      GMBUS_SLAVE_READ | GMBUS_SW_RDY);
>   	while (len) {
> @@ -389,9 +407,16 @@ gmbus_xfer_read_chunk(struct drm_i915_private *dev_priv,
>   
>   		val = I915_READ_FW(GMBUS3);
>   		do {
> +			if (extra_byte_added && len == 1)
> +				break;
> +
>   			*buf++ = val & 0xff;
>   			val >>= 8;
>   		} while (--len && ++loop < 4);
> +
> +		if (burst_read && len == size - 4)
> +			/* Reset the override bit */
> +			I915_WRITE_FW(GMBUS0, gmbus0_reg);
>   	}
>   
>   	return 0;
> @@ -399,7 +424,7 @@ gmbus_xfer_read_chunk(struct drm_i915_private *dev_priv,
>   
>   static int
>   gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
> -		u32 gmbus1_index)
> +		u32 gmbus0_reg, u32 gmbus1_index)
>   {
>   	u8 *buf = msg->buf;
>   	unsigned int rx_size = msg->len;
> @@ -407,10 +432,13 @@ gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
>   	int ret;
>   
>   	do {
> -		len = min(rx_size, gmbus_max_xfer_size(dev_priv));
> +		if (HAS_GMBUS_BURST_READ(dev_priv))
> +			len = rx_size;
> +		else
> +			len = min(rx_size, gmbus_max_xfer_size(dev_priv));
>   
> -		ret = gmbus_xfer_read_chunk(dev_priv, msg->addr,
> -					    buf, len, gmbus1_index);
> +		ret = gmbus_xfer_read_chunk(dev_priv, msg->addr, buf, len,
> +					    gmbus0_reg, gmbus1_index);
>   		if (ret)
>   			return ret;
>   
> @@ -498,7 +526,8 @@ gmbus_is_index_xfer(struct i2c_msg *msgs, int i, int num)
>   }
>   
>   static int
> -gmbus_index_xfer(struct drm_i915_private *dev_priv, struct i2c_msg *msgs)
> +gmbus_index_xfer(struct drm_i915_private *dev_priv, struct i2c_msg *msgs,
> +		 u32 gmbus0_reg)
>   {
>   	u32 gmbus1_index = 0;
>   	u32 gmbus5 = 0;
> @@ -516,7 +545,8 @@ gmbus_index_xfer(struct drm_i915_private *dev_priv, struct i2c_msg *msgs)
>   		I915_WRITE_FW(GMBUS5, gmbus5);
>   
>   	if (msgs[1].flags & I2C_M_RD)
> -		ret = gmbus_xfer_read(dev_priv, &msgs[1], gmbus1_index);
> +		ret = gmbus_xfer_read(dev_priv, &msgs[1], gmbus0_reg,
> +				      gmbus1_index);
>   	else
>   		ret = gmbus_xfer_write(dev_priv, &msgs[1], gmbus1_index);
>   
> @@ -551,10 +581,12 @@ do_gmbus_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs, int num,
>   	for (; i < num; i += inc) {
>   		inc = 1;
>   		if (gmbus_is_index_xfer(msgs, i, num)) {
> -			ret = gmbus_index_xfer(dev_priv, &msgs[i]);
> +			ret = gmbus_index_xfer(dev_priv, &msgs[i],
> +					       gmbus0_source | bus->reg0);
>   			inc = 2; /* an index transmission is two msgs */
>   		} else if (msgs[i].flags & I2C_M_RD) {
> -			ret = gmbus_xfer_read(dev_priv, &msgs[i], 0);
> +			ret = gmbus_xfer_read(dev_priv, &msgs[i],
> +					      gmbus0_source | bus->reg0, 0);
>   		} else {
>   			ret = gmbus_xfer_write(dev_priv, &msgs[i], 0);
>   		}

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