linux-kernel - Re: [RFC, PATCH] HSI: cmt_speech: fix timestamp interface

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Date:	Fri, 10 Apr 2015 15:36:10 +0200
From:	Sebastian Reichel <sre@...nel.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Kai Vehmanen <kvehmanen@...ignal.fi>,
	Aaro Koskinen <aaro.koskinen@....fi>,
	Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: [RFC, PATCH] HSI: cmt_speech: fix timestamp interface

Hi Arnd,

[CC'd Kai's new mail address]

On Fri, Apr 10, 2015 at 01:59:19PM +0200, Arnd Bergmann wrote:
> The user interface for timestamps in the new cms_speech
> driver is broken in multiple ways:

ow :( Thanks for the report.

> - The layout is incompatible between 32-bit and 64-bit user
>   space, because of the size differences in 'struct timespec'.
>   This means that the driver can not work when used with 32-bit
>   user space on a 64-bit kernel.
> 
> - As there are plans to change 32-bit user space to use
>   a 64-bit time_t type in the future, it will also be
>   incompatible with new 32-bit user space.
> 
> As the driver is not yet widely used, now would be a good time
> to change it to just use a 64-bit nanosecond value, which also
> happens to be more efficient. In order to make the layout
> non-ambiguous, we have to also add extra padding so the 64-bit
> value is naturally aligned.
> 
> The change requires the respective update to user space tools
> using it. If that is for some reason not possible any more,
> another solution would be to replace the timespec structure
> with two __u32 values to avoid incompatibilities later.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

I think updating to two __u32 values is the better solution here.
While the driver is new in the mainline kernel it has been used
for multiple years downstream in the Maemo/MeeGo/Mer communities
and I would prefer to keep userspace compatibility.

-- Sebastian

> diff --git a/drivers/hsi/clients/cmt_speech.c b/drivers/hsi/clients/cmt_speech.c
> index 4983529a9c6c..568dbc117f2e 100644
> --- a/drivers/hsi/clients/cmt_speech.c
> +++ b/drivers/hsi/clients/cmt_speech.c
> @@ -451,9 +451,7 @@ static void cs_hsi_read_on_control_complete(struct hsi_msg *msg)
>  	dev_dbg(&hi->cl->device, "Read on control: %08X\n", cmd);
>  	cs_release_cmd(msg);
>  	if (hi->flags & CS_FEAT_TSTAMP_RX_CTRL) {
> -		struct timespec *tstamp =
> -			&hi->mmap_cfg->tstamp_rx_ctrl;
> -		do_posix_clock_monotonic_gettime(tstamp);
> +		hi->mmap_cfg->tstamp_rx_ctrl = ktime_get_ns();
>  	}
>  	spin_unlock(&hi->lock);
>  
> diff --git a/include/uapi/linux/hsi/cs-protocol.h b/include/uapi/linux/hsi/cs-protocol.h
> index 4957bba57cbe..fce633c7c649 100644
> --- a/include/uapi/linux/hsi/cs-protocol.h
> +++ b/include/uapi/linux/hsi/cs-protocol.h
> @@ -90,12 +90,12 @@ struct cs_mmap_config_block {
>  	__u32 tx_offsets[CS_MAX_BUFFERS];
>  	__u32 rx_ptr;
>  	__u32 rx_ptr_boundary;
> -	__u32 reserved3[2];
> +	__u32 reserved3[3];
>  	/*
>  	 * if enabled with CS_FEAT_TSTAMP_RX_CTRL, monotonic
>  	 * timestamp taken when the last control command was received
>  	 */
> -	struct timespec tstamp_rx_ctrl;
> +	__u64 tstamp_rx_ctrl;
>  };
>  
>  #define CS_IO_MAGIC		'C'

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