[PATCH v3 1/2] drm/dp: Factor out a function to probe a DPCD address

[Jani Nikula]

On Mon, 11 Apr 2022, Imre Deak <imre.deak at intel.com> wrote:
> Factor out from drm_dp_dpcd_read() a function to probe a DPCD address
> with a 1-byte read access. This will be needed by the next patch doing a
> read from an LTTPR address, which must happen without the preceding
> wake-up read in drm_dp_dpcd_read().
>
> While at it add tracing for the 1 byte read even if the read was
> successful.

Let's see if that's going to be annoying and confusing...

>
> v2: Add a probe function instead of exporting drm_dp_dpcd_access(). (Jani)
> v3: Add tracing for the 1-byte read even if the read was successful. (Khaled)
>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Cc: Khaled Almahallawy <khaled.almahallawy at intel.com>
> Cc: dri-devel at lists.freedesktop.org
> Signed-off-by: Imre Deak <imre.deak at intel.com>

Reviewed-by: Jani Nikula <jani.nikula at intel.com>

> ---
>  drivers/gpu/drm/dp/drm_dp.c    | 33 ++++++++++++++++++++++++++++-----
>  include/drm/dp/drm_dp_helper.h |  1 +
>  2 files changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/dp/drm_dp.c b/drivers/gpu/drm/dp/drm_dp.c
> index 580016a1b9eb7..2a1f5ff6633cc 100644
> --- a/drivers/gpu/drm/dp/drm_dp.c
> +++ b/drivers/gpu/drm/dp/drm_dp.c
> @@ -527,6 +527,31 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
>  	return ret;
>  }
>  
> +/**
> + * drm_dp_dpcd_probe() - probe a given DPCD address with a 1-byte read access
> + * @aux: DisplayPort AUX channel (SST)
> + * @offset: address of the register to probe
> + *
> + * Probe the provided DPCD address by reading 1 byte from it. The function can
> + * be used to trigger some side-effect the read access has, like waking up the
> + * sink, without the need for the read-out value.
> + *
> + * Returns 0 if the read access suceeded, or a negative error code on failure.
> + */
> +int drm_dp_dpcd_probe(struct drm_dp_aux *aux, unsigned int offset)
> +{
> +	u8 buffer;
> +	int ret;
> +
> +	ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset, &buffer, 1);
> +	WARN_ON(ret == 0);
> +
> +	drm_dp_dump_access(aux, DP_AUX_NATIVE_READ, offset, &buffer, ret);
> +
> +	return ret < 0 ? ret : 0;
> +}
> +EXPORT_SYMBOL(drm_dp_dpcd_probe);
> +
>  /**
>   * drm_dp_dpcd_read() - read a series of bytes from the DPCD
>   * @aux: DisplayPort AUX channel (SST or MST)
> @@ -559,10 +584,9 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
>  	 * monitor doesn't power down exactly after the throw away read.
>  	 */
>  	if (!aux->is_remote) {
> -		ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, DP_DPCD_REV,
> -					 buffer, 1);
> -		if (ret != 1)
> -			goto out;
> +		ret = drm_dp_dpcd_probe(aux, DP_DPCD_REV);
> +		if (ret < 0)
> +			return ret;
>  	}
>  
>  	if (aux->is_remote)
> @@ -571,7 +595,6 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
>  		ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset,
>  					 buffer, size);
>  
> -out:
>  	drm_dp_dump_access(aux, DP_AUX_NATIVE_READ, offset, buffer, ret);
>  	return ret;
>  }
> diff --git a/include/drm/dp/drm_dp_helper.h b/include/drm/dp/drm_dp_helper.h
> index 1eccd97419436..91af98e6617c6 100644
> --- a/include/drm/dp/drm_dp_helper.h
> +++ b/include/drm/dp/drm_dp_helper.h
> @@ -2053,6 +2053,7 @@ struct drm_dp_aux {
>  	bool is_remote;
>  };
>  
> +int drm_dp_dpcd_probe(struct drm_dp_aux *aux, unsigned int offset);
>  ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
>  			 void *buffer, size_t size);
>  ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,

-- 
Jani Nikula, Intel Open Source Graphics Center