Re: [PATCH 6/9] nvme-apple: Add initial Apple SoC NVMe driver

From: Keith Busch <kbusch@kernel.org>
To: Sven Peter <sven@svenpeter.dev>
Cc: Jens Axboe <axboe@fb.com>, Christoph Hellwig <hch@lst.de>,
	Sagi Grimberg <sagi@grimberg.me>,
	Hector Martin <marcan@marcan.st>,
	Alyssa Rosenzweig <alyssa@rosenzweig.io>,
	Rob Herring <robh+dt@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Marc Zyngier <maz@kernel.org>,
	devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org
Subject: Re: [PATCH 6/9] nvme-apple: Add initial Apple SoC NVMe driver
Date: Mon, 21 Mar 2022 11:01:39 -0600	[thread overview]
Message-ID: <Yjivcz4BlCMfJBkM@C02CK6Q3MD6M> (raw)
In-Reply-To: <20220321165049.35985-7-sven@svenpeter.dev>

On Mon, Mar 21, 2022 at 05:50:46PM +0100, Sven Peter wrote:
> +static bool apple_nvme_poll_cq(struct apple_nvme_queue *q,
> +			       struct io_comp_batch *iob)
> +{
> +	bool found = false;
> +
> +	while (apple_nvme_cqe_pending(q)) {
> +		found = true;
> +
> +		/*
> +		 * load-load control dependency between phase and the rest of
> +		 * the cqe requires a full read memory barrier
> +		 */
> +		dma_rmb();
> +		apple_nvme_handle_cqe(q, iob, q->cq_head);
> +		apple_nvme_update_cq_head(q);
> +	}
> +
> +	if (found)
> +		writel_relaxed(q->cq_head, q->cq_db);

Doesn't a relaxed write mean that a subsequent write can bypass the previous?
If so, that sounds like it can corrupt the cq head.