Re: [PATCH v2 1/2] drm: Add retries for dp dual mode read

[Imre Deak <imre.deak@intel.com>]

On Wed, Aug 23, 2017 at 06:12:51PM +0530, Shashank Sharma wrote:
> From the CI builds, its been observed that during a driver
> reload/insert, dp dual mode read function sometimes fails to
> read from dual mode devices (like LSPCON) over i2c-over-aux
> channel.
> 
> This patch:
> - adds some delay and few retries, allowing a scope for these
>   devices to settle down and respond.
> - changes one error log's level from ERROR->DEBUG as we want
>   to call it an error only after all the retries are exhausted.
> 
> V2: Addressed review comments from Jani (for loop for retry)
> 
> Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
> Cc: Imre Deak <imre.deak@intel.com>
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
> ---
>  drivers/gpu/drm/drm_dp_dual_mode_helper.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_dual_mode_helper.c b/drivers/gpu/drm/drm_dp_dual_mode_helper.c
> index 80e62f6..09bf962 100644
> --- a/drivers/gpu/drm/drm_dp_dual_mode_helper.c
> +++ b/drivers/gpu/drm/drm_dp_dual_mode_helper.c
> @@ -75,8 +75,16 @@ ssize_t drm_dp_dual_mode_read(struct i2c_adapter *adapter,
>  		},
>  	};
>  	int ret;
> +	int retry;
> +
> +	for (retry = 0; retry < 6; retry++) {
> +		if (retry)
> +			usleep_range(500, 1000);
> +		ret = i2c_transfer(adapter, msgs, ARRAY_SIZE(msgs));
> +		if (ret > 0)

I think the ret == 0 case should be handled with the rest of
partial-read cases (which we check against further down). So the above
should be
		if (ret >= 0)

With this fixed it looks ok to me, so fwiw:
Reviewed-by: Imre Deak <imre.deak@intel.com>

Note that ideally we wouldn't retry for error returns like
-ENOTSUPPORTED, -ENOMEM, -E2BIG, but it's not a big deal to do that
either. One possibility to solving that and also making this workaround
more generic is to reuse the existing retry logic in __i2c_transfer() by
making it retry in all relevant error cases (timeout/IO) and setting
i2c_adapter::retries for the LSPCON adaptor. But that can be done as a
follow-up.

> +			break;
> +	}
>  
> -	ret = i2c_transfer(adapter, msgs, ARRAY_SIZE(msgs));
>  	if (ret < 0)
>  		return ret;
>  	if (ret != ARRAY_SIZE(msgs))
> @@ -420,7 +428,7 @@ int drm_lspcon_get_mode(struct i2c_adapter *adapter,
>  	ret = drm_dp_dual_mode_read(adapter, DP_DUAL_MODE_LSPCON_CURRENT_MODE,
>  				    &data, sizeof(data));
>  	if (ret < 0) {
> -		DRM_ERROR("LSPCON read(0x80, 0x41) failed\n");
> +		DRM_DEBUG_KMS("LSPCON read(0x80, 0x41) failed\n");
>  		return -EFAULT;
>  	}
>  
> -- 
> 2.7.4
> 
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