Re: [PATCH 4/8] drm: Wait 1ms before retrying aux transactions on EBUSY.

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

On Sat, 21 Nov 2015, Rodrigo Vivi <rodrigo.vivi@intel.com> wrote:
> DP Specs isn't really clear about the amount of retries,
> but for cases it mentions retries it also mention times that
> vary from 300us to 1ms.
>
> For many cases hardware can handled the timeouts before retry
> is possible and allowed, but for many other cases it is better
> to wait and give time so the aux channels can recover.
>
> For instance one general case there is when downstream device
> is waking up from sleep states generating HPD so it might take
> up to 1ms before getting responsive.
>
> I believe with this msleep we could minimize the 32 times retries
> and still let Dell monitors happy, but I don't have this monitor
> to test here so let's just add the sleep for now and still retry
> 32 times.
>
> Cc: Dave Airlie <airlied@redhat.com>
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index ee7c955..1d6016d 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -202,9 +202,11 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
>  			if (err == -EAGAIN)
>  				continue;
>  
> -			/* FIXME: On BUSY we could wait before retrying */
> -			if (err == -EBUSY)
> +			/* Give a time for aux channels to recover */
> +			if (err == -EBUSY) {
> +				msleep(1);

usleep_range please; msleep(1) may take *much* longer than 1 ms, and
that could throw us off with our retry logic.

BR,
Jani.

>  				continue;
> +			}
>  
>  			return err;
>  		}

-- 
Jani Nikula, Intel Open Source Technology Center
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