linux-kernel - Re: [RFC PATCH v1 22/57] sound: Remove PAGE

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Message-ID: <5e9ee10b-fc15-4c6a-a666-0d80ec8bdd9a@arm.com>
Date: Mon, 14 Oct 2024 13:52:36 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Takashi Iwai <tiwai@...e.de>
Cc: Mark Brown <broonie@...nel.org>, Andrew Morton
 <akpm@...ux-foundation.org>, Anshuman Khandual <anshuman.khandual@....com>,
 Ard Biesheuvel <ardb@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 David Hildenbrand <david@...hat.com>, Greg Marsden
 <greg.marsden@...cle.com>, Ivan Ivanov <ivan.ivanov@...e.com>,
 Jaroslav Kysela <perex@...ex.cz>, Kalesh Singh <kaleshsingh@...gle.com>,
 Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Matthias Brugger <mbrugger@...e.com>, Miroslav Benes <mbenes@...e.cz>,
 Takashi Iwai <tiwai@...e.com>, Will Deacon <will@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-sound@...r.kernel.org
Subject: Re: [RFC PATCH v1 22/57] sound: Remove PAGE_SIZE compile-time
 constant assumption

On 14/10/2024 13:41, Takashi Iwai wrote:
> 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.

OK great I'll set these to 4096 and 4096*2 for the next version.

Thanks for the feedback!

> 
> 
> thanks,
> 
> Takashi