[Intel-gfx] [PATCH] drm/i915: Add delay on DPCD reads

[Daniel Vetter]

On Tue, Mar 15, 2016 at 01:38:58PM +0200, Mika Kahola wrote:
> Additional 50 ms delay is needed between DPCD reads on HP Bizlink 1326
> DP to VGA adapter. Having said that, I haven't noticed a need for
> additional delay between DPCD reads on other DP-VGA dongles.
> 
> While at it, let's replace mdelay() with usleep_range() routine.
> 
> Signed-off-by: Mika Kahola <mika.kahola at intel.com>

Please don't add more hacks to our own private copy of read_wake. This
stuff needs to move into drm core, since really we can't be the only ones
who try to plug in shit dongles ;-)

Would be great to test this with an amd DP card and the exact same
connector and see what happens. Then move these read_wake hacks into the
drm_dp_dpcd_read function. Lyude is also working on something like this,
but for DP MST hubs.
-Daniel

> ---
>  drivers/gpu/drm/i915/intel_dp.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 0e326e7..ff3883c 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -3125,9 +3125,11 @@ intel_dp_dpcd_read_wake(struct drm_dp_aux *aux, unsigned int offset,
>  
>  	for (i = 0; i < 3; i++) {
>  		ret = drm_dp_dpcd_read(aux, offset, buffer, size);
> -		if (ret == size)
> +		if (ret == size) {
> +			usleep_range(50000, 50100);
>  			return ret;
> -		msleep(1);
> +		}
> +		usleep_range(1000, 1100);
>  	}
>  
>  	return ret;
> -- 
> 1.9.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch