[PATCH v2] drm/display: Don't assume dual mode adaptors support i2c sub-addressing

[Ville Syrjälä]

On Thu, Oct 06, 2022 at 11:33:14AM +0200, Simon Rettberg wrote:
> Current dual mode adaptor ("DP++") detection code assumes that all
> adaptors support i2c sub-addressing for read operations from the
> DP-HDMI adaptor ID buffer.  It has been observed that multiple
> adaptors do not in fact support this, and always return data starting
> at register 0.  On affected adaptors, the code fails to read the proper
> registers that would identify the device as a type 2 adaptor, and
> handles those as type 1, limiting the TMDS clock to 165MHz, even if
> the according register would announce a higher TMDS clock.
> Fix this by always reading the ID buffer starting from offset 0, and
> discarding any bytes before the actual offset of interest.
> 
> We tried finding authoritative documentation on whether or not this is
> allowed behaviour, but since all the official VESA docs are paywalled,
> the best we could come up with was the spec sheet for Texas Instruments'
> SNx5DP149 chip family.[1]  It explicitly mentions that sub-addressing is
> supported for register writes, but *not* for reads (See NOTE in
> section 8.5.3).  Unless TI openly decided to violate the VESA spec, one
> could take that as a hint that sub-addressing is in fact not mandated
> by VESA.
> The other two adaptors affected used the PS8409(A) and the LT8611,
> according to the data returned from their ID buffers.
> 
> [1] https://www.ti.com/lit/ds/symlink/sn75dp149.pdf
> 
> Signed-off-by: Simon Rettberg <simon.rettberg at rz.uni-freiburg.de>
> Reviewed-by: Rafael Gieschke <rafael.gieschke at rz.uni-freiburg.de>
> ---
> 
> v2 changes form last submission's feedback (thanks for taking the time):
> - Added a shortened version of our "background story" to the commit message
> - Only use tmpbuf if the read offset is != 0

Bounced to intel-gfx to get the i915 CI to check it...

> 
>  .../gpu/drm/display/drm_dp_dual_mode_helper.c | 51 +++++++++++--------
>  1 file changed, 29 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c b/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c
> index 3ea53bb67d3b..bd61e20770a5 100644
> --- a/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c
> @@ -63,23 +63,45 @@
>  ssize_t drm_dp_dual_mode_read(struct i2c_adapter *adapter,
>  			      u8 offset, void *buffer, size_t size)
>  {
> +	u8 zero = 0;
> +	char *tmpbuf = NULL;
> +	/*
> +	 * As sub-addressing is not supported by all adaptors,
> +	 * always explicitly read from the start and discard
> +	 * any bytes that come before the requested offset.
> +	 * This way, no matter whether the adaptor supports it
> +	 * or not, we'll end up reading the proper data.
> +	 */
>  	struct i2c_msg msgs[] = {
>  		{
>  			.addr = DP_DUAL_MODE_SLAVE_ADDRESS,
>  			.flags = 0,
>  			.len = 1,
> -			.buf = &offset,
> +			.buf = &zero,
>  		},
>  		{
>  			.addr = DP_DUAL_MODE_SLAVE_ADDRESS,
>  			.flags = I2C_M_RD,
> -			.len = size,
> +			.len = size + offset,
>  			.buf = buffer,
>  		},
>  	};
>  	int ret;
>  
> +	if (offset) {
> +		tmpbuf = kmalloc(size + offset, GFP_KERNEL);
> +		if (!tmpbuf)
> +			return -ENOMEM;
> +
> +		msgs[1].buf = tmpbuf;
> +	}
> +
>  	ret = i2c_transfer(adapter, msgs, ARRAY_SIZE(msgs));
> +	if (tmpbuf)
> +		memcpy(buffer, tmpbuf + offset, size);
> +
> +	kfree(tmpbuf);
> +
>  	if (ret < 0)
>  		return ret;
>  	if (ret != ARRAY_SIZE(msgs))
> @@ -208,18 +230,6 @@ enum drm_dp_dual_mode_type drm_dp_dual_mode_detect(const struct drm_device *dev,
>  	if (ret)
>  		return DRM_DP_DUAL_MODE_UNKNOWN;
>  
> -	/*
> -	 * Sigh. Some (maybe all?) type 1 adaptors are broken and ack
> -	 * the offset but ignore it, and instead they just always return
> -	 * data from the start of the HDMI ID buffer. So for a broken
> -	 * type 1 HDMI adaptor a single byte read will always give us
> -	 * 0x44, and for a type 1 DVI adaptor it should give 0x00
> -	 * (assuming it implements any registers). Fortunately neither
> -	 * of those values will match the type 2 signature of the
> -	 * DP_DUAL_MODE_ADAPTOR_ID register so we can proceed with
> -	 * the type 2 adaptor detection safely even in the presence
> -	 * of broken type 1 adaptors.
> -	 */
>  	ret = drm_dp_dual_mode_read(adapter, DP_DUAL_MODE_ADAPTOR_ID,
>  				    &adaptor_id, sizeof(adaptor_id));
>  	drm_dbg_kms(dev, "DP dual mode adaptor ID: %02x (err %zd)\n", adaptor_id, ret);
> @@ -233,11 +243,10 @@ enum drm_dp_dual_mode_type drm_dp_dual_mode_detect(const struct drm_device *dev,
>  				return DRM_DP_DUAL_MODE_TYPE2_DVI;
>  		}
>  		/*
> -		 * If neither a proper type 1 ID nor a broken type 1 adaptor
> -		 * as described above, assume type 1, but let the user know
> -		 * that we may have misdetected the type.
> +		 * If not a proper type 1 ID, still assume type 1, but let
> +		 * the user know that we may have misdetected the type.
>  		 */
> -		if (!is_type1_adaptor(adaptor_id) && adaptor_id != hdmi_id[0])
> +		if (!is_type1_adaptor(adaptor_id))
>  			drm_err(dev, "Unexpected DP dual mode adaptor ID %02x\n", adaptor_id);
>  
>  	}
> @@ -343,10 +352,8 @@ EXPORT_SYMBOL(drm_dp_dual_mode_get_tmds_output);
>   * @enable: enable (as opposed to disable) the TMDS output buffers
>   *
>   * Set the state of the TMDS output buffers in the adaptor. For
> - * type2 this is set via the DP_DUAL_MODE_TMDS_OEN register. As
> - * some type 1 adaptors have problems with registers (see comments
> - * in drm_dp_dual_mode_detect()) we avoid touching the register,
> - * making this function a no-op on type 1 adaptors.
> + * type2 this is set via the DP_DUAL_MODE_TMDS_OEN register.
> + * Type1 adaptors do not support any register writes.
>   *
>   * Returns:
>   * 0 on success, negative error code on failure
> -- 
> 2.35.1

-- 
Ville Syrjälä
Intel