linux-kernel - Re: [PATCH v5 4/5] Driver core: platform: Add devm_platform_get_irqs

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Date:   Wed, 9 Dec 2020 19:36:52 +0000
From:   John Garry <john.garry@...wei.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     <jejb@...ux.ibm.com>, <martin.petersen@...cle.com>,
        <lenb@...nel.org>, <rjw@...ysocki.net>, <tglx@...utronix.de>,
        <maz@...nel.org>, <linux-scsi@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linuxarm@...wei.com>,
        <linux-acpi@...r.kernel.org>, <dwagner@...e.de>
Subject: Re: [PATCH v5 4/5] Driver core: platform: Add
 devm_platform_get_irqs_affinity()

On 09/12/2020 19:13, Greg KH wrote:

Hi Greg,

>> For this HW version, the device is on the system bus, directly addressable
>> by the CPU.
> What do you mean by "system bus"?

Maybe my terminology is wrong, the point is that we have a platform 
device driver.

> 
>> Motivation is that I wanted to switch the HW completion queues to use
>> managed interrupts.
> Fair enough, seems like overkill for a "platform" bus though:)
> 
>>> What in-kernel driver needs this complexity?  I can't take new apis
>>> without a real user in the tree, sorry.
>> It's in the final patch in the serieshttps://lore.kernel.org/linux-scsi/1606905417-183214-1-git-send-email-john.garry@huawei.com/T/#m0df7e7cd6f0819b99aaeb6b7f8939ef1e17b8a83.
> Ah, I missed that, I thought that was some high-speed scsi thing, not a
> tiny platform driver...

It is actually is a high-speed SCSI thing also, SAS 3.0 :)

> 
>> I don't anticipate a huge number of users of this API in future, as most
>> multi-queue devices are PCI devices; so we could do the work of this API in
>> the driver itself, but the preference was not to export genirq functions
>> like irq_update_affinity_desc() or irq_create_affinity_masks(), and rather
>> have a common helper in the core platform code.
> Ok, I'd like to have the irq maintainers/developers ack this before
> taking it in the driver core, as someone is going to have to maintain
> this crazy thing for forever if it gets merged.
> 

irq experts are cc'ed and have been very helpful here

So the API mushroomed a bit over time, as I realized that we need to 
support tearing down the irq mapping, make as devm method, use 
irq_calc_affinity_vectors(). Not sure how we could factor any of it out 
to become less of your problem.

Thanks,
John