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: <87wm9frf5x.wl-tiwai@suse.de>
Date: Fri, 13 Jun 2025 14:37:14 +0200
From: Takashi Iwai <tiwai@...e.de>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Takashi Iwai <tiwai@...e.de>,
	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 Fri, 13 Jun 2025 13:03:04 +0200,
Christophe Leroy wrote:
> 
> 
> 
> Le 13/06/2025 à 11:29, Takashi Iwai a écrit :
> > 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.
> 
> Shall I understand you prefer this series over the more simple "ALSA:
> pcm: Convert snd_pcm_ioctl_sync_ptr_{compat/x32} to
> user_access_begin/user_access_end()" patch ?

Err, no, sorry for ambiguity.
I wanted to move the replacement of tstamp_sec/nsec with struct
__snd_timespec as a small preliminary patch from patch#3.
That is,
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -3103,11 +3103,9 @@ struct snd_pcm_mmap_status32 {
 	snd_pcm_state_t state;
 	s32 pad1;
 	u32 hw_ptr;
-	s32 tstamp_sec;
-	s32 tstamp_nsec;
+	struct __snd_timespec tstamp;
 	snd_pcm_state_t suspended_state;
-	s32 audio_tstamp_sec;
-	s32 audio_tstamp_nsec;
+	struct __snd_timespec audio_tstamp;
 } __packed;
etc.  By factoring this out, it becomes clear that the timespec
compatibility is fully cared.

__snd_timespec may be defined in different ways on user-space, but in
the kernel code, it's a single definition of s32/s32 pair.  This needs
to be emphasized.


thanks,

Takashi