Re: [RFC PATCH 3/3] ALSA: pcm: Convert snd_pcm_sync_ptr() to user_access_begin/user_access_end()

[Christophe Leroy]

Le 13/06/2025 à 14:37, Takashi Iwai a écrit :
> 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@xxxxxxxxxx>
> > >
> > > 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.

Then I'm lost.

I sent two alternative proposals:
A/ Single patch, simple, handling only two fonctions 
snd_pcm_ioctl_sync_ptr_{compat/x32} , without refactoring. [1]
B/ This RFC series, more elaborate, refactoring and putting user copy 
into helper macros. [2]

So the question was to be sure you prefer alternative B over alternative 
A. I guess the answer is YES as you asking me improve it.

[1] 
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/8df11af98033e4cb4d9b0f16d6e9d5b69110b036.1749724057.git.christophe.leroy@xxxxxxxxxx/
[2] 
https://patchwork.ozlabs.org/project/linuxppc-dev/list/?state=*&series=460665


> I wanted to move the replacement of tstamp_sec/nsec with struct
> __snd_timespec as a small preliminary patch from patch#3.
> That is,

Yes that's what I understood.

Thanks
Christophe


> --- 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