linux-kernel - Re: [QUEUED v20180920 05/16] stm class: Add a helper for writing data packets

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Date:   Thu, 27 Sep 2018 11:07:48 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [QUEUED v20180920 05/16] stm class: Add a helper for writing
 data packets

On Thu, Sep 20, 2018 at 03:45:42PM +0300, Alexander Shishkin wrote:
> Add a helper to write a sequence of bytes as STP data packets. This
> is used by protocol drivers to output their metadata, as well as the
> actual data payload.
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> ---
>  drivers/hwtracing/stm/core.c | 50 ++++++++++++++++++++++++++----------
>  drivers/hwtracing/stm/stm.h  |  3 +++
>  2 files changed, 40 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/hwtracing/stm/core.c b/drivers/hwtracing/stm/core.c
> index c869a30661ac..9ed2f06deb47 100644
> --- a/drivers/hwtracing/stm/core.c
> +++ b/drivers/hwtracing/stm/core.c
> @@ -564,27 +564,51 @@ stm_assign_first_policy(struct stm_device *stm, struct stm_output *output,
>  	return err;
>  }
>  
> -static ssize_t notrace stm_write(struct stm_data *data, unsigned int master,
> -			  unsigned int channel, const char *buf, size_t count)
> +/**
> + * stm_data_write() - send the given payload as data packets
> + * @data:	stm driver's data
> + * @m:		STP master
> + * @c:		STP channel
> + * @ts_first:	timestamp the first packet
> + * @buf:	data payload buffer
> + * @count:	data payload size
> + */
> +ssize_t notrace stm_data_write(struct stm_data *data, unsigned int m,
> +			       unsigned int c, bool ts_first, const void *buf,
> +			       size_t count)
>  {
> -	unsigned int flags = STP_PACKET_TIMESTAMPED;
> -	const unsigned char *p = buf, nil = 0;
> -	size_t pos;
> +	unsigned int flags = ts_first ? STP_PACKET_TIMESTAMPED : 0;
>  	ssize_t sz;
> +	size_t pos;
>  
> -	for (pos = 0, p = buf; count > pos; pos += sz, p += sz) {
> +	for (pos = 0, sz = 0; pos < count; pos += sz) {
>  		sz = min_t(unsigned int, count - pos, 8);
> -		sz = data->packet(data, master, channel, STP_PACKET_DATA, flags,
> -				  sz, p);
> -		flags = 0;
> -
> -		if (sz < 0)
> +		sz = data->packet(data, m, c, STP_PACKET_DATA, flags, sz,
> +				  &((u8 *)buf)[pos]);
> +		if (sz <= 0)
>  			break;
> +
> +		if (ts_first) {
> +			flags = 0;
> +			ts_first = false;
> +		}
>  	}
>  
> -	data->packet(data, master, channel, STP_PACKET_FLAG, 0, 0, &nil);
> +	return sz < 0 ? sz : pos;
> +}
> +EXPORT_SYMBOL_GPL(stm_data_write);
> +
> +static ssize_t notrace stm_write(struct stm_data *data, unsigned int master,
> +			  unsigned int channel, const char *buf, size_t count)
> +{
> +	ssize_t sz;
> +
> +	sz = stm_data_write(data, master, channel, true, buf, count);
> +	if (sz > 0)
> +		data->packet(data, master, channel, STP_PACKET_FLAG, 0, 0,
> +			     buf);

The original code the payload of a flag packet was '0' while in this patch
changes it to be anything.  Some external tooling could be very confused.

>  
> -	return pos;
> +	return sz;
>  }
>  
>  static ssize_t stm_char_write(struct file *file, const char __user *buf,
> diff --git a/drivers/hwtracing/stm/stm.h b/drivers/hwtracing/stm/stm.h
> index 921ebd9fd3bd..753d83acb7ae 100644
> --- a/drivers/hwtracing/stm/stm.h
> +++ b/drivers/hwtracing/stm/stm.h
> @@ -111,5 +111,8 @@ int stm_lookup_protocol(const char *name,
>  			const struct stm_protocol_driver **pdrv,
>  			const struct config_item_type **type);
>  void stm_put_protocol(const struct stm_protocol_driver *pdrv);
> +ssize_t stm_data_write(struct stm_data *data, unsigned int m,
> +		       unsigned int c, bool ts_first, const void *buf,
> +		       size_t count);
>  
>  #endif /* _STM_STM_H_ */
> -- 
> 2.18.0
>