Re: [BUG] SD card reader disappears after suspend

From: Alan Stern <stern@rowland.harvard.edu>
To: Samuel Sadok <samuel.sadok@bluewin.ch>
Cc: USB list <linux-usb@vger.kernel.org>,
	Mathias Nyman <mathias.nyman@intel.com>, <yu.c.chen@intel.com>,
	<mironov.ivan@gmail.com>
Subject: Re: [BUG] SD card reader disappears after suspend
Date: Mon, 13 Jan 2020 10:15:02 -0500 (EST)	[thread overview]
Message-ID: <Pine.LNX.4.44L0.2001131006400.1502-100000@iolanthe.rowland.org> (raw)
In-Reply-To: <CAJ4e7SX=vJXx1=AQ+f9ajJK_BBgzV-u_vr9V+uLffyv4+vO+ug@mail.gmail.com>

On Mon, 13 Jan 2020, Samuel Sadok wrote:

> Hello,
> 
> It's been a while that there was any discussion on this bug so here's
> a brief context: There's a bug on some MacBook Pro's where the
> built-in SD card reader would be missing after a suspend/resume cycle.
> 
> After looking at the schematics for my laptop I noticed that
> `OC6*/GPIO10` of the Intel PCH is connected to a net called
> `SD_PWR_EN`. So I tried to set this pin to 0 and then to 1 again in
> order to completely power cycle the card reader. Turns out this
> finally makes the card reader appear after a suspend/resume cycle.
> 
> For the record, here's the user space workaround:
> https://gist.github.com/samuelsadok/6d7b3e3015d3370a92ed4702e4d3c4b5#gistcomment-3134437
> 
> Are there other buggy devices for which the kernel already implements
> such a power cycle? Otherwise, where would be the right place to add
> this?

This is a platform-specific solution, so it belongs in a
platform-specific region of the kernel.  Any necessary changes would
mostly have to go outside the USB stack -- after all, GPIO lines aren't
USB devices.

If the kernel already contains some system-setup code specific to the 
MacBook Pro, that's where such a fix belongs.

> Additional observations:
>  - by default (after boot), GPIO10 is configured as input and shows
> the state "high". In the schematics there's a pull-up resistor. and I
> assume it goes to a mosfet (off-sheet). So the problem is not that the
> card reader has no power after suspend. It just needs a power cycle.
>  - the card reader is connected to the PCH via the four USB3 data
> lines, but the two USB2 data lines are not connected. Is this a legal
> configuration? Might this prevent the card reader from receiving power
> state commands?

It is not allowed for a generic connection (i.e., to an external
device) but I think it's okay for something that's built into the
platform.  It could prevent the card reader from working correctly if
for some reason the USB-3 signalling failed and the device tried to
switch over to USB-2 signalling.

On the other hand, there's no real reason I can think of for the card 
reader to need a power cycle following resume.  Fixing that problem, 
whatever it is, would be a better solution.

Alan Stern