linux-kernel - Re: [PATCH 4/5] auxdisplay: linedisp: support attribute attachment to auxdisplay devices

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Message-ID: <CB8AD4ED-5E01-49EB-A177-9C7431E87181@gmail.com>
Date: Wed, 03 Sep 2025 07:31:05 -0400
From: Jean-François Lessard <jefflessard3@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
CC: Andy Shevchenko <andy@...nel.org>, Geert Uytterhoeven <geert@...ux-m68k.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] auxdisplay: linedisp: support attribute attachment to auxdisplay devices

Le 3 septembre 2025 06 h 18 min 18 s HAE, Andy Shevchenko <andriy.shevchenko@...el.com> a écrit :
>On Tue, Sep 02, 2025 at 01:37:52PM -0400, Jean-François Lessard wrote:
>> Le 2 septembre 2025 06 h 18 min 24 s HAE, Andy Shevchenko <andriy.shevchenko@...el.com> a écrit :
>> >On Sun, Aug 31, 2025 at 10:00:28PM -0400, Jean-François Lessard wrote:
>
>...
>
>> >> +static DEFINE_SPINLOCK(linedisp_attachments_lock);
>> >
>> >Why spin lock and not mutex?
>> 
>> The attachment list operations are extremely lightweight (just adding/removing
>> list entries), making spinlock the optimal choice because:
>> - Very short critical sections: Only list traversal and pointer assignments;
>>   avoids context switching overhead for brief operations
>> - No sleeping operations: No memory allocation or I/O within locked sections
>> - Future-proof atomic context safety: Can be safely called from interrupt
>>   handlers if needed
>
>To me it sounds like solving non-existing problem. I am sure we will see no
>driver that tries to call this API in an atomic context.
>

Yeah, I should have skipped "Future-proof atomic context safety".
I don't see how attach/detach would be called from atomic context.
Maybe to_linedisp(), but not in the current implementation anyway.