Re: [RFC PATCH v1 22/57] sound: Remove PAGE_SIZE compile-time constant assumption

[Takashi Iwai <tiwai@suse.de>]

On Mon, 14 Oct 2024 14:24:02 +0200,
Ryan Roberts wrote:
> 
> On 14/10/2024 12:38, Mark Brown wrote:
> > On Mon, Oct 14, 2024 at 11:58:29AM +0100, Ryan Roberts wrote:
> >> -static const struct snd_pcm_hardware dummy_dma_hardware = {
> >> +static DEFINE_GLOBAL_PAGE_SIZE_VAR_CONST(struct snd_pcm_hardware, dummy_dma_hardware, {
> >>  	/* Random values to keep userspace happy when checking constraints */
> >>  	.info			= SNDRV_PCM_INFO_INTERLEAVED |
> >>  				  SNDRV_PCM_INFO_BLOCK_TRANSFER,
> >> @@ -107,7 +107,7 @@ static const struct snd_pcm_hardware dummy_dma_hardware = {
> >>  	.period_bytes_max	= PAGE_SIZE*2,
> >>  	.periods_min		= 2,
> >>  	.periods_max		= 128,
> >> -};
> >> +});
> > 
> > It's probably better to just use PAGE_SIZE_MAX here and avoid the
> > deferred patching, like the comment says we don't particularly care what
> > the value actually is here given that it's a dummy.
> 
> OK, so would that be:
> 
> 	.buffer_bytes_max	= 128*1024,
> 	.period_bytes_min	= PAGE_SIZE_MAX,      <<<<<
> 	.period_bytes_max	= PAGE_SIZE_MAX*2,    <<<<<
> 	.periods_min		= 2,
> 	.periods_max		= 128,
> 
> ?
> 
> It's not really clear to me how all the parameters interact; the buffer size
> 128K, which, if PAGE_SIZE_MAX is 64K, would hold 1 period of the maximum size.
> But periods_min is 2. So not sure that works? Or perhaps I'm trying to apply too
> much meaning to the param names...

Right, when PAGE_SIZE_MAX is 64k, 128k won't be used because of the
constrant of periods_min=2.

As Mark mentioned, here the actual size itself doesn't matter much.
So I suppose it'd be even simpler to define just 4096 and 4096 * 2 for
period_bytes_min and *_max instead of sticking with PAGE_SIZE.  Then
it would become platform-agnostic, too.


thanks,

Takashi