Re: [PATCH v3 2/2] ALSA: control: add ioctl to retrieve full card components

Takashi Iwai
2026-03-09T10:03:10Z

On Mon, 09 Mar 2026 10:44:25 +0100,
Jaroslav Kysela wrote:
>
> On 3/6/26 11:43, Takashi Iwai wrote:
> > On Fri, 06 Mar 2026 10:39:46 +0100,
> > Jaroslav Kysela wrote:
> >>
> >> On 3/5/26 11:18, Takashi Iwai wrote:
> >>> On Thu, 05 Mar 2026 11:11:40 +0100,
> >>> Maciej Strozek wrote:
> >>>>
> >>>> W dniu czw, 05.03.2026 o godzinie 11∶04 +0100, użytkownik Takashi Iwai
> >>>> napisał:
> >>>>> On Thu, 05 Mar 2026 10:54:35 +0100,
> >>>>> Maciej Strozek wrote:
> >>>>>>
> >>>>>> W dniu wto, 03.03.2026 o godzinie 16∶47 +0100, użytkownik Takashi
> >>>>>> Iwai
> >>>>>> napisał:
> >>>>>>>>
> >>>>>>>> + */
> >>>>>>>> +struct snd_ctl_card_components {
> >>>>>>>> + int card;
> >>>>>>>> + unsigned int length;
> >>>>>>>> + unsigned char *components;
> >>>>>>>> +};
> >>>>>>>
> >>>>>>> And the ioctl can serve for two purposes:
> >>>>>>>
> >>>>>>> - When length=0 is set, the kernel stores the current number of
> >>>>>>> bytes
> >>>>>>>   and returns without copying.  User-space can use this mode for
> >>>>>>>   allocating the buffer.
> >>>>>>>
> >>>>>> In alsa-lib all data must be allocated beforehand, so this
> >>>>>> length==0
> >>>>>> query is not very useful there, it will just go into a [512] array
> >>>>>> anyway. Are there any other users that may benefit from this?
> >>>>>
> >>>>> My suggested API can work even with the fixed size 512, too, if 512
> >>>>> is
> >>>>> hight enough.  It's just more flexible.  And there is no restriction
> >>>>> about alsa-lib data allocation; the function can query the size then
> >>>>> allocate, too.
> >>>>>
> >>>>>
> >>>>> Takashi
> >>>>
> >>>> OK, will prepare v4 with this, thanks
> >>>
> >>> Well, let's see how others think, too. The API design needs more
> >>> considerations because we can't change it any longer once after
> >>> defined.
> >>
> >> I think that the indirect pointer in ioctl structure is the best at
> >> the moment unless we decide to use the fixed char array.
> >
> > OK, it might be indeed better if the user-space API is something like:
> >
> > int snd_ctl_card_components(snd_ctl_t *ctl, unsigned char *buf, size_t len);
> >
> > Then it's simpler to pass the pointer as is without copying.
> >
> >> But (for
> >> discussion) we may try to be a bit clever and define universal bytes
> >> ioctl which may carry also other things in future like:
> >>
> >> enum {
> >> SND_CTL_CARD_BTYPE_COMPONENTS = 1
> >> };
> >
> > So this is for future extensions?
> >
> >> struct snd_ctl_card_bytes {
> >> unsigned int card; // this is duplication with info ioctl
> >> // to be removed?
> >
> > Right, it sounds like superfluous. I thought we were to allow
> > extracting a card info for a different card number, but it doesn't
> > look so.
> >
> >> unsigned int type; // e.g. SND_CTL_CARD_BTYPE_COMPONENTS
> >> unsigned int data_allocated; // overall size of data
> >> unsigned int data_len; // actual data len
> >> unsigned char *data; // pointer to data array
> >> };
> >>
> >> Scenarios:
> >>
> >> data_allocated = 0 or data == NULL -> driver just returns data_len
> >> data_allocated < data_len -> driver returns -ENOMEM
> >> data_allocated >= data_len -> driver will copy data
> >>
> >> Note that data_len will be zero from the user space for read
> >> operations (driver knows it). But we can eventually use this ioctl to
> >> set some data in future, so data_len/data will be used for the write
> >> operation.
> >
> > In all cases, data_len is filled with the expected data size in
> > return, right?
>
> I would return data_len only when data_allocated == 0 or when the user
> space array can hold complete data. When ioctl returns an error code
> (e.g. ENOMEM), the structure should not be modified IMHO.

I find it OK, otherwise you'd need one more ioctl, but it's a kind of
bike-shedding topic, and I don't mind much whether it should be so or
not.

> Eventually, we can extend the structure to be even more universal and
> add 'data_offset' and 'data_overall_len' to support fully partial
> transfers. In this case, data_len would mean filled/used chunk size
> and the "overflow" error won't exist.

Yeah, we can, and I had that in mind, too. But it makes things
complex, so let's not step into it yet.

thanks,

Takashi