[PATCH RFC v3 1/7] drm/display: dp: implement new access helpers

[Lyude Paul]

A few tiny nitpicks below, but with those addressed:

Reviewed-by: Lyude Paul <lyude at redhat.com>

On Fri, 2025-03-07 at 06:34 +0200, Dmitry Baryshkov wrote:
> From: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
> 
> Existing DPCD access functions return an error code or the number of
> bytes being read / write in case of partial access. However a lot of
> drivers either (incorrectly) ignore partial access or mishandle error
> codes. In other cases this results in a boilerplate code which compares
> returned value with the size.
> 
> Implement new set of DPCD access helpers, which ignore partial access,
> always return 0 or an error code.
> 
> Suggested-by: Jani Nikula <jani.nikula at linux.intel.com>
> Acked-by: Jani Nikula <jani.nikula at intel.com>
> Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
> ---
>  drivers/gpu/drm/display/drm_dp_helper.c |  4 ++
>  include/drm/display/drm_dp_helper.h     | 92 ++++++++++++++++++++++++++++++++-
>  2 files changed, 94 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> index dbce1c3f49691fc687fee2404b723c73d533f23d..e43a8f4a252dae22eeaae1f4ca94da064303033d 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -704,6 +704,8 @@ EXPORT_SYMBOL(drm_dp_dpcd_set_powered);
>   * function returns -EPROTO. Errors from the underlying AUX channel transfer
>   * function, with the exception of -EBUSY (which causes the transaction to
>   * be retried), are propagated to the caller.
> + *
> + * In most of the cases you want to use drm_dp_dpcd_read_data() instead.
>   */
>  ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
>  			 void *buffer, size_t size)
> @@ -752,6 +754,8 @@ EXPORT_SYMBOL(drm_dp_dpcd_read);
>   * function returns -EPROTO. Errors from the underlying AUX channel transfer
>   * function, with the exception of -EBUSY (which causes the transaction to
>   * be retried), are propagated to the caller.
> + *
> + * In most of the cases you want to use drm_dp_dpcd_write_data() instead.
>   */
>  ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,
>  			  void *buffer, size_t size)
> diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
> index 5ae4241959f24e2c1fb581d7c7d770485d603099..c5be44d72c9a04474f6c795e03bf02bf08f5eaef 100644
> --- a/include/drm/display/drm_dp_helper.h
> +++ b/include/drm/display/drm_dp_helper.h
> @@ -527,6 +527,64 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
>  ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,
>  			  void *buffer, size_t size);
>  
> +/**
> + * drm_dp_dpcd_read_data() - read a series of bytes from the DPCD
> + * @aux: DisplayPort AUX channel (SST or MST)
> + * @offset: address of the (first) register to read
> + * @buffer: buffer to store the register values
> + * @size: number of bytes in @buffer
> + *
> + * Returns zero (0) on success, or a negative error
> + * code on failure. -EIO is returned if the request was NAKed by the sink or
> + * if the retry count was exceeded. If not all bytes were transferred, this
> + * function returns -EPROTO. Errors from the underlying AUX channel transfer
> + * function, with the exception of -EBUSY (which causes the transaction to
> + * be retried), are propagated to the caller.
> + */
> +static inline int drm_dp_dpcd_read_data(struct drm_dp_aux *aux,
> +					unsigned int offset,
> +					void *buffer, size_t size)
> +{
> +	int ret;
> +
> +	ret = drm_dp_dpcd_read(aux, offset, buffer, size);
> +	if (ret < 0)
> +		return ret;
> +	if (ret < size)
> +		return -EPROTO;
> +
> +	return 0;
> +}
> +
> +/**
> + * drm_dp_dpcd_write_data() - write a series of bytes to the DPCD
> + * @aux: DisplayPort AUX channel (SST or MST)
> + * @offset: address of the (first) register to write
> + * @buffer: buffer containing the values to write
> + * @size: number of bytes in @buffer
> + *
> + * Returns zero (0) on success, or a negative error
> + * code on failure. -EIO is returned if the request was NAKed by the sink or
> + * if the retry count was exceeded. If not all bytes were transferred, this
> + * function returns -EPROTO. Errors from the underlying AUX channel transfer
> + * function, with the exception of -EBUSY (which causes the transaction to
> + * be retried), are propagated to the caller.
> + */
> +static inline int drm_dp_dpcd_write_data(struct drm_dp_aux *aux,
> +					 unsigned int offset,
> +					 void *buffer, size_t size)
> +{
> +	int ret;
> +
> +	ret = drm_dp_dpcd_write(aux, offset, buffer, size);
> +	if (ret < 0)
> +		return ret;
> +	if (ret < size)
> +		return -EPROTO;
> +
> +	return 0;
> +}
> +
>  /**
>   * drm_dp_dpcd_readb() - read a single byte from the DPCD
>   * @aux: DisplayPort AUX channel
> @@ -534,7 +592,8 @@ ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,
>   * @valuep: location where the value of the register will be stored
>   *
>   * Returns the number of bytes transferred (1) on success, or a negative
> - * error code on failure.
> + * error code on failure. In most of the cases you should be using
> + * drm_dp_dpcd_read_byte() instead

Missing a period here

>   */
>  static inline ssize_t drm_dp_dpcd_readb(struct drm_dp_aux *aux,
>  					unsigned int offset, u8 *valuep)
> @@ -549,7 +608,8 @@ static inline ssize_t drm_dp_dpcd_readb(struct drm_dp_aux *aux,
>   * @value: value to write to the register
>   *
>   * Returns the number of bytes transferred (1) on success, or a negative
> - * error code on failure.
> + * error code on failure. In most of the cases you should be using
> + * drm_dp_dpcd_write_byte() instead

And here

Otherwise looks great! :)

>   */
>  static inline ssize_t drm_dp_dpcd_writeb(struct drm_dp_aux *aux,
>  					 unsigned int offset, u8 value)
> @@ -557,6 +617,34 @@ static inline ssize_t drm_dp_dpcd_writeb(struct drm_dp_aux *aux,
>  	return drm_dp_dpcd_write(aux, offset, &value, 1);
>  }
>  
> +/**
> + * drm_dp_dpcd_read_byte() - read a single byte from the DPCD
> + * @aux: DisplayPort AUX channel
> + * @offset: address of the register to read
> + * @valuep: location where the value of the register will be stored
> + *
> + * Returns zero (0) on success, or a negative error code on failure.
> + */
> +static inline int drm_dp_dpcd_read_byte(struct drm_dp_aux *aux,
> +					unsigned int offset, u8 *valuep)
> +{
> +	return drm_dp_dpcd_read_data(aux, offset, valuep, 1);
> +}
> +
> +/**
> + * drm_dp_dpcd_write_byte() - write a single byte to the DPCD
> + * @aux: DisplayPort AUX channel
> + * @offset: address of the register to write
> + * @value: value to write to the register
> + *
> + * Returns zero (0) on success, or a negative error code on failure.
> + */
> +static inline int drm_dp_dpcd_write_byte(struct drm_dp_aux *aux,
> +					 unsigned int offset, u8 value)
> +{
> +	return drm_dp_dpcd_write_data(aux, offset, &value, 1);
> +}
> +
>  int drm_dp_read_dpcd_caps(struct drm_dp_aux *aux,
>  			  u8 dpcd[DP_RECEIVER_CAP_SIZE]);
>  
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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