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Date:   Tue, 14 May 2019 08:11:41 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Mario.Limonciello@dell.com
Cc:     hch@lst.de, kbusch@kernel.org, keith.busch@intel.com,
        sagi@grimberg.me, linux-nvme@lists.infradead.org,
        rafael@kernel.org, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, kai.heng.feng@canonical.com
Subject: Re: [PATCH] nvme/pci: Use host managed power state for suspend
Message-ID: <20190514061141.GA7059@lst.de>
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On Mon, May 13, 2019 at 06:01:39PM +0000, Mario.Limonciello@dell.com wrote:
> When using HMB the SSD will be writing to some memory mapped region.
> Writing to
> that region would use DMA to access host memory, no?

Memory mapped region?  It will use the devices DMA engine to write
host memory, which we explicitly allowed it.

> If the DMA controller is not functional writing to that region won't work properly as 
> it can't access that memory.

The DMA controller is in the device.  External DMA controllers are only
used on very low-end periphals, usually cheap IP blocks like SPI
controllers or similar.