linux-kernel - Re: [PATCH v4 2/3] ALSA: compress_offload: Add SNDRV_COMPRESS

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Message-ID: <aKMzGRzeRqkOwTNS@opensource.cirrus.com>
Date: Mon, 18 Aug 2025 15:05:13 +0100
From: Charles Keepax <ckeepax@...nsource.cirrus.com>
To: Takashi Iwai <tiwai@...e.de>
Cc: Vinod Koul <vkoul@...nel.org>, Joris Verhaegen <verhaegen@...gle.com>,
        Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
        Liam Girdwood <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>,
        Cezary Rojewski <cezary.rojewski@...el.com>,
        Peter Ujfalusi <peter.ujfalusi@...ux.intel.com>,
        Bard Liao <yung-chuan.liao@...ux.intel.com>,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        Kai Vehmanen <kai.vehmanen@...ux.intel.com>,
        Pierre-Louis Bossart <pierre-louis.bossart@...ux.dev>,
        Srinivas Kandagatla <srini@...nel.org>,
        Daniel Baluta <daniel.baluta@....com>,
        Orson Zhai <orsonzhai@...il.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>,
        Masami Hiramatsu <mhiramat@...nel.org>, kernel-team@...roid.com,
        linux-sound@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...nsource.cirrus.com, linux-arm-msm@...r.kernel.org,
        sound-open-firmware@...a-project.org,
        linux-arm-kernel@...ts.infradead.org,
        Miller Liang <millerliang@...gle.com>
Subject: Re: [PATCH v4 2/3] ALSA: compress_offload: Add
 SNDRV_COMPRESS_TSTAMP64 ioctl

On Tue, Aug 05, 2025 at 07:59:42AM +0200, Takashi Iwai wrote:
> On Tue, 05 Aug 2025 06:47:59 +0200,
> Vinod Koul wrote:
> > On 01-08-25, 10:27, Joris Verhaegen wrote:
> > >  	ret = snd_compr_update_tstamp(stream, &tstamp64);
> > >  	if (ret == 0) {
> > > -		snd_compr_tstamp32_from_64(&tstamp32, &tstamp64);
> > > -		ret = copy_to_user((struct snd_compr_tstamp __user *)arg,
> > > -				   &tstamp32, sizeof(tstamp32)) ?
> > > +		if (is_32bit) {
> > > +			snd_compr_tstamp32_from_64(&tstamp32, &tstamp64);
> > > +			copy_from = &tstamp32;
> > > +			copy_size = sizeof(tstamp32);
> > > +		}
> > 
> > Most of the applications and people would be 32bit right now and we
> > expect this to progressively change, but then this imposes a penalty as
> > default path is 64 bit, since we expect this ioctl to be called very
> > frequently, should we do this optimization for 64bit here?
> 
> Through a quick glance over the patch, I don't think you'll hit the
> significant performance loss.  It's merely a few bytes of extra copies
> before copy_to_user(), after all.  But, of course, it'd be more
> convincing if anyone can test and give the actual numbers.
> 

I am inclined to agree the impact should be very minimal and the
only alternative is a more complex implementation. I would vote
for leaving this as is.

Thanks,
Charles