Re: [PATCH 1/3] drm: Add retries for lspcon status check

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 22 Aug 2017, Shashank Sharma <shashank.sharma@intel.com> wrote:
> It's an observation during some CI tests that few LSPCON chips
> respond slow while system is under load, and need some delay
> while reading current mode status using i2c-over-aux channel.
>
> This patch:
> - Adds few retries and delays before declaring a read
>   failure from LSPCON hardware.
> - Changes the debug level of the print from ERROR->DEBUG
>   whereas another patch in I915 will add an ERROR message
>   from the caller when we have timed out all our limits.
>
> V2: addressed review comments from Imre, and added r-b
>     - use int instead of u8 for counter
>     - use for loop instead of do...while();
> V3: Rebase
>
> Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
> Cc: Imre Deak <imre.deak@intel.com>
>
> Reviewed-by: Imre Deak <imre.deak@intel.com>
> Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
> Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
> ---
>  drivers/gpu/drm/drm_dp_dual_mode_helper.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 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..fc8c7ac 100644
> --- a/drivers/gpu/drm/drm_dp_dual_mode_helper.c
> +++ b/drivers/gpu/drm/drm_dp_dual_mode_helper.c
> @@ -409,6 +409,7 @@ int drm_lspcon_get_mode(struct i2c_adapter *adapter,
>  			enum drm_lspcon_mode *mode)
>  {
>  	u8 data;
> +	int retry;
>  	int ret = 0;
>  
>  	if (!mode) {
> @@ -417,10 +418,17 @@ int drm_lspcon_get_mode(struct i2c_adapter *adapter,
>  	}
>  
>  	/* Read Status: i2c over aux */
> -	ret = drm_dp_dual_mode_read(adapter, DP_DUAL_MODE_LSPCON_CURRENT_MODE,
> -				    &data, sizeof(data));
> +	for (retry = 5; ; retry--) {
> +		ret = drm_dp_dual_mode_read(adapter,
> +					    DP_DUAL_MODE_LSPCON_CURRENT_MODE,
> +					    &data, sizeof(data));
> +		if (!ret || !retry)
> +			break;
> +		usleep_range(500, 1000);
> +	}

Sorry, but that looks like a programming quiz to me. At most how many
time does drm_dp_dual_mode_read() get called?

With this, for example, the C programmer's spine tells you "six times"
for the paradigm for loop before you even really stop to think about it:

	for (try = 0; try < 6; try++) {
        	if (try)
                	usleep_range(500, 1000);
                ret = drm_dp_dual_mode_read();
                if (!ret)
                	break;
        }

BR,
Jani.


PS. I'm left wondering if "retry = 5" was there to emphasize that
there's one try and five *retries*.



> +
>  	if (ret < 0) {
> -		DRM_ERROR("LSPCON read(0x80, 0x41) failed\n");
> +		DRM_DEBUG_KMS("LSPCON read(0x80, 0x41) failed\n");
>  		return -EFAULT;
>  	}

-- 
Jani Nikula, Intel Open Source Technology Center
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