linux-kernel - Re: [RFC PATCH 3/3] ALSA: pcm: Convert snd_pcm_sync_ptr() to user_access_begin/user_access

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Message-ID: <878qlwrnv1.wl-tiwai@suse.de>
Date: Fri, 13 Jun 2025 11:29:22 +0200
From: Takashi Iwai <tiwai@...e.de>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Jaroslav Kysela <perex@...ex.cz>,
	Takashi Iwai <tiwai@...e.com>,
	linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	linux-sound@...r.kernel.org,
	Herve Codina <herve.codina@...tlin.com>,
	Mark Brown <broonie@...nel.org>
Subject: Re: [RFC PATCH 3/3] ALSA: pcm: Convert snd_pcm_sync_ptr() to user_access_begin/user_access_end()

On Thu, 12 Jun 2025 12:51:05 +0200,
Christophe Leroy wrote:
> 
> Now that snd_pcm_sync_ptr_get_user() and snd_pcm_sync_ptr_put_user()
> are converted to user_access_begin/user_access_end(),
> snd_pcm_sync_ptr_get_user() is more efficient than a raw get_user()
> followed by a copy_from_user(). And because copy_{to/from}_user() are
> generic functions focussed on transfer of big data blocks to/from user,
> snd_pcm_sync_ptr_put_user() is also more efficient for small amont of
> data.
> 
> So use snd_pcm_sync_ptr_get_user() and snd_pcm_sync_ptr_put_user() in
> snd_pcm_sync_ptr() too.
> 
> In order to have snd_pcm_mmap_status32 similar to snd_pcm_mmap_status,
> replace to tsamp_{sec/nsec} and audio_tstamp_{sec/nsec} by equivalent
> struct __snd_timespec.
> 
> snd_pcm_ioctl_sync_ptr_buggy() is left as it is because the conversion
> wouldn't be straigh-forward do to the workaround it provides.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>

Through a quick glance, all patches look almost fine, but one favor to
ask: this patch contains the convert from s32/s32 pair to struct
__snd_timespec.  It should be factored out to a prerequisite patch
instead of burying in a big change.

I'm asking it because this timepsec definition is very confusing (and
complex) due to historical reasons, and it should be handled with a
special care.
IIUC, struct __snd_timespec is always s32/s32 for the kernel code, so
the conversion must be fine.  This needs to be commented in the
commit.


Thanks!

Takashi