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Date:   Sun, 26 Apr 2020 23:14:26 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     rui_feng@realsil.com.cn
Cc:     arnd@arndb.de, gregkh@linuxfoundation.org, ulf.hansson@linaro.org,
        linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org
Subject: Re: [PATCH] mmc: rtsx: Add SD Express mode support for RTS5261
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On Sun, Apr 26, 2020 at 09:25:46AM +0800, rui_feng@realsil.com.cn wrote:
> From: Rui Feng <rui_feng@realsil.com.cn>
> 
> RTS5261 support legacy SD mode and SD Express mode.
> In SD7.x, SD association introduce SD Express as a new mode.
> SD Express mode is distinguished by CMD8.
> Therefore, CMD8 has new bit for SD Express.
> SD Express is based on PCIe/NVMe.
> RTS5261 uses CMD8 to switch to SD Express mode.

So how does this bit work?  They way I imagined SD Express to work
is that the actual SD Card just shows up as a real PCIe device, similar
to say Thunderbolt.