From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paul Burton Subject: Re: [REGRESSION] "console: don't prefer first registered if DT specifies stdout-path" breaks console on video outputs of various ARM boards Date: Sat, 5 Nov 2016 11:00:23 +0000 Message-ID: <2806714.DJA9EBBcvx@np-p-burton> References: <20161104121135.4780-1-hdegoede@redhat.com> <17833033.PtOSTzKXGD@np-p-burton> Mime-Version: 1.0 Content-Type: multipart/signed; boundary="nextPart1801633.8lPRXRx3SU"; micalg=pgp-sha256; protocol="application/pgp-signature" Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Hans de Goede Cc: Linus Torvalds , Andrew Morton , Rob Herring , Frank Rowand , Thorsten Leemhuis , Greg Kroah-Hartman , Tejun Heo , devicetree@vger.kernel.org, linux-kernel@vger.kernel.org List-Id: devicetree@vger.kernel.org --nextPart1801633.8lPRXRx3SU Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" On Friday, 4 November 2016 14:22:17 GMT Hans de Goede wrote: > Hi, > > On 04-11-16 13:30, Paul Burton wrote: > > Hi Hans, > > > > On Friday, 4 November 2016 13:11:34 GMT Hans de Goede wrote: > >> Hi All, > >> > >> While booting 4.9-rc# for the first time on an Allwinner A33 tablet, > >> I noticed that after u-boot the LCD display stayed black. It turns out > >> that there was an issue which caused X to never get up, and all kernel > >> (and other startup) messages prior to that only went to ttyS0 which > >> consists of 2 tiny testpads on the PCB with this tablet. > >> > >> The same issue will also happen on any ARM boards which have a HDMI or > >> composite video output and which use a stdout-path pointing to their > >> serial console. I think this will e.g. also impact the Raspberry Pi, > >> I know for certain that this will impact the 99 different Allwinnner > >> boards currently supported by mainline u-boot + the mainline kernel. > >> > >> This is a behavior changes from previous kernels and I consider this > >> a regression. Thus I propose to revert the commit in question, for more > >> info here is a partial copy of the commit message of the proposed revert: > >> > >> The reverted commit changes existing behavior on which many ARM boards > >> rely. Many ARM small-board-computers, like e.g. the Raspberry Pi have > >> both a video output and a serial console. Depending on whether the user > >> is using the device as a more regular computer; or as a headless device > >> we need to have the console on either one or the other. > >> > >> Many users rely on the kernel behavior of the console being present on > >> both outputs, before the reverted commit the console setup with no > >> console= kernel arguments on an ARM board which sets stdout-path in dt > >> would look like this: > >> > >> [root@localhost ~]# cat /proc/consoles > >> ttyS0 -W- (EC p a) 4:64 > >> tty0 -WU (E p ) 4:1 > >> > >> Where as after the reverted commit, it looks like this: > >> > >> [root@localhost ~]# cat /proc/consoles > >> ttyS0 -W- (EC p a) 4:64 > >> > >> This commit reverts commit 05fd007e4629 ("console: don't prefer first > >> registered if DT specifies stdout-path") restoring the original behavior. > >> > >> Regards, > >> > >> Hans > > > > Ugh... so the devices you're talking about rely upon set stdout-path in > > their device tree but effectively rely upon us ignoring it? > > No they rely on the kernel using stdout-path as an extra console while > keeping tty0 as console, not ignoring it. This how stdout-path has always > worked (at least as long as the Allwinner boards have used it, which has > been 2 - 3 years now). > > If you want only the console specified by stdout-path you can get this by > specifying it with console=... on the kernel cmdline. > > > If that's the case then I guess reverting is probably the best option, but > > it does restore us to a position where we honor stdout-path for earlycon > > & then essentially ignore it for the proper kernel console. That seems > > pretty bust to me... > > We do not ignore it, we use both the tty pointed to by stdout-path and tty0. > > Regards, > > Hans Hi Hans, Could you walk me though how you're getting that behaviour from the current code? I don't see how that would happen besides perhaps if drivers are probed in a fortunate order. Is that what you're relying upon? What I see in my systems, and what 05fd007e4629 ("console: don't prefer first registered if DT specifies stdout-path") addressed, is that if there are for example 2 UARTs uart0 & uart1 that are probed in that order and stdout-path indicates that we should use uart1 we wind up essentially ignoring it because the ordering of the relevant calls goes: - of_console_check() for uart0 - add_preferred_console() for uart0 - register_console() for uart0 - of_console_check() for uart1 - add_preferred_console() for uart1 - register_console() for uart1 Since of_check_console() doesn't get called for uart1 until after uart0 has been probed, we don't add an entry for it to the console_cmdline array until after register_console() has already decided to enable uart0 because preferred_console == -1. I'm not the only one seeing this oddity either, for example see the discussion on this patch: https://patchwork.kernel.org/patch/9263753/ By simply reverting my patch you restore us to a position where so far as I can see we simply do not honor stdout-path for the real kernel console. Thanks, Paul --nextPart1801633.8lPRXRx3SU Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part. 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