linux-kernel - Re: [PATCH 2/2] soundwire: bus: Add internal slave ID and use for IRQs

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Message-ID: <dc069aec-8f34-4745-804b-936e0466559c@linux.dev>
Date: Fri, 25 Apr 2025 18:35:27 +0200
From: Pierre-Louis Bossart <pierre-louis.bossart@...ux.dev>
To: Charles Keepax <ckeepax@...nsource.cirrus.com>
Cc: vkoul@...nel.org, yung-chuan.liao@...ux.intel.com,
 sanyog.r.kale@...el.com, linux-sound@...r.kernel.org,
 linux-kernel@...r.kernel.org, patches@...nsource.cirrus.com
Subject: Re: [PATCH 2/2] soundwire: bus: Add internal slave ID and use for
 IRQs

On 4/22/25 14:11, Charles Keepax wrote:
> On Tue, Apr 22, 2025 at 01:50:13PM +0200, Pierre-Louis Bossart wrote:
>> On 4/22/25 12:42, Charles Keepax wrote:
>> I see the patch add a limit during probe, so now that
>> means the number of ACPI devices MUST be lower than 11. That
>> doesn't sound right to me and could cause some configurations
>> to fail. It's perfectly ok to have ghost devices and no limits
>> on how many our BIOS colleagues decide to list.
> 
> Yeah it will limit the ACPI to 11 devices. I can't say I am a
> huge fan of the "ghost" devices, like its ACPI it is for
> describing the hardware, so it should describe the hardware.
> 
> That said my thinking on this was I have not seen a system with
> more than 4 real devices on a single bus, and not more than a
> couple ghosts in the ACPI. So this didn't look like a big issue.
> 
>> Using a dedicated IDA for IRQ mapping looks like a good
>> idea to me, but I don't think you can really use the same IDA
>> for dev_num
> 
> If you are really concerned about the ghost devices I could back
> out the part that reuses the ID for the dev_num. However I do
> need to know the maximum number of devices when the IRQ domain is
> allocated. So I can't really see a way to avoid picking a maximum
> number of devices for the bus. What number of real + ghosts would
> you be comfortable with? Although I guess if not using it for the
> dev_num it is then fairly easy to expand if needed so perhaps I
> just back out the dev_num bit, but stick with 11 for now and we
> can expand if necessary?

A maximum of 16 devices total is probably ok. That's 10 more than the worst-case configuration we've seen so far, and I can't think of a case where more than 10 ghost devices would be listed.