[alsa-devel] [PATCH 1/3] ALSA: pcm: implement the anonymous dup (inode file descriptor)

[Takashi Iwai]

On Tue, 29 Jan 2019 18:59:07 +0100,
Jaroslav Kysela wrote:
> 
> This patch implements new SNDRV_PCM_IOCTL_ANONYMOUS_DUP ioctl which
> returns the new duplicated anonymous inode file descriptor
> (anon_inode:snd-pcm) which can be passed to the restricted clients.
> 
> This patch is meant to be the alternative for the dma-buf interface. Both
> implementation have some pros and cons:
> 
> anon_inode:dmabuf
> 
> - a bit standard export API for the DMA buffers
> - fencing for the concurrent access [1]
> - driver/kernel interface for the DMA buffer [1]
> - multiple attach/detach scheme [1]
> 
> [1] the real usage for the sound PCM is unknown at the moment for this feature
> 
> anon_inode:snd-pcm
> 
> - simple (no problem with ref-counting, non-standard mmap implementation etc.)
> - allow to use more sound interfaces for the file descriptor like status ioctls
> - more fine grained security policies (another anon_inode name unshared with
>   other drivers)

Your sign-off seems missing.


> @@ -999,15 +1002,19 @@ int snd_pcm_attach_substream(struct snd_pcm *pcm, int stream,
>  	}
>  
>  	if (file->f_flags & O_APPEND) {
> -		if (prefer_subdevice < 0) {
> -			if (pstr->substream_count > 1)
> -				return -EINVAL; /* must be unique */
> -			substream = pstr->substream;
> +		if (clone) {
> +			substream = clone;
>  		} else {
> -			for (substream = pstr->substream; substream;
> -			     substream = substream->next)
> -				if (substream->number == prefer_subdevice)
> -					break;
> +			if (prefer_subdevice < 0) {
> +				if (pstr->substream_count > 1)
> +					return -EINVAL; /* must be unique */
> +				substream = pstr->substream;
> +			} else {
> +				for (substream = pstr->substream; substream;
> +				     substream = substream->next)
> +					if (substream->number == prefer_subdevice)
> +						break;
> +			}
>  		}
>  		if (! substream)
>  			return -ENODEV;

So the clone case should return via this block, then...

> @@ -1018,11 +1025,18 @@ int snd_pcm_attach_substream(struct snd_pcm *pcm, int stream,
>  		return 0;
>  	}
>  
> -	for (substream = pstr->substream; substream; substream = substream->next) {
> -		if (!SUBSTREAM_BUSY(substream) &&
> -		    (prefer_subdevice == -1 ||
> -		     substream->number == prefer_subdevice))
> -			break;
> +	if (clone) {
> +		substream = clone;
> +		if (SUBSTREAM_BUSY(substream))
> +			return -EAGAIN;
> +	} else {
> +		for (substream = pstr->substream; substream;
> +		     substream = substream->next) {
> +			if (!SUBSTREAM_BUSY(substream) &&
> +			    (prefer_subdevice == -1 ||
> +			     substream->number == prefer_subdevice))
> +				break;
> +		}

... do we need to support cloning without O_APPEND?


thanks,

Takashi