linux-kernel - Re: [PATCH v3] nvme-pci: Support shared tags across queues for Apple 2018 controllers

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Message-ID: <2030a028664a9af9e96fffca3ab352faf1f739e5.camel@kernel.crashing.org>
Date:   Tue, 30 Jul 2019 13:28:57 -0700
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Keith Busch <kbusch@...nel.org>
Cc:     linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Paul Pawlowski <paul@...rm.io>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Minwoo Im <minwoo.im.dev@...il.com>,
        Damien Le Moal <Damien.LeMoal@....com>
Subject: Re: [PATCH v3] nvme-pci: Support shared tags across queues for
 Apple 2018 controllers

On Tue, 2019-07-30 at 09:30 -0600, Keith Busch wrote:
> On Fri, Jul 19, 2019 at 03:31:02PM +1000, Benjamin Herrenschmidt wrote:
> > From 8dcba2ef5b1466b023b88b4eca463b30de78d9eb Mon Sep 17 00:00:00 2001
> > From: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > Date: Fri, 19 Jul 2019 15:03:06 +1000
> > Subject: 
> > 
> > Another issue with the Apple T2 based 2018 controllers seem to be
> > that they blow up (and shut the machine down) if there's a tag
> > collision between the IO queue and the Admin queue.
> > 
> > My suspicion is that they use our tags for their internal tracking
> > and don't mix them with the queue id. They also seem to not like
> > when tags go beyond the IO queue depth, ie 128 tags.
> > 
> > This adds a quirk that marks tags 0..31 of the IO queue reserved
> > 
> > Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > ---
> 
> One problem is that we've an nvme parameter, io_queue_depth, that a user
> could set to something less than 32, and then you won't be able to do
> any IO. I'd recommend enforce the admin queue to QD1 for this device so
> that you have more potential IO tags.

So I had a look and it's not that trivial. I would have to change
a few things that use constants for the admin queue depth, such as
the AEN tag etc...

For such a special case, I am tempted instead to do the much simpler:

	if (dev->ctrl.quirks & NVME_QUIRK_SHARED_TAGS) {
		if (dev->q_depth < (NVME_AQ_DEPTH + 2))
			dev->q_depth = NVME_AQ_DEPTH + 2;
	}

In nvme_pci_enable() next to the existing q_depth hackery for other
controllers.

Thoughts ?

Cheers,
Ben.