linux-kernel - Re: [PATCH v2 05/13] usb: musb: create debugfs directory under usb root

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Date:   Fri, 15 Nov 2019 07:45:30 -0600
From:   Bin Liu <b-liu@...com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Felipe Balbi <balbi@...nel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Peter Chen <Peter.Chen@....com>,
        Minas Harutyunyan <hminas@...opsys.com>,
        Cristian Birsan <cristian.birsan@...rochip.com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>,
        Kevin Cernekee <cernekee@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        <bcm-kernel-feedback-list@...adcom.com>,
        Daniel Mack <daniel@...que.org>,
        Haojian Zhuang <haojian.zhuang@...il.com>,
        Robert Jarzmik <robert.jarzmik@...e.fr>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Stephen Boyd <swboyd@...omium.org>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        Colin Ian King <colin.king@...onical.com>,
        Biju Das <biju.das@...renesas.com>,
        Fabrizio Castro <fabrizio.castro@...renesas.com>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Yangtao Li <tiny.windzz@...il.com>,
        <linux-media@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-usb@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH v2 05/13] usb: musb: create debugfs directory under usb
 root

On Fri, Nov 15, 2019 at 11:21:53AM +0800, Greg Kroah-Hartman wrote:
> On Thu, Nov 14, 2019 at 08:02:34AM -0600, Bin Liu wrote:
> > On Thu, Nov 14, 2019 at 11:20:18AM +0800, Greg Kroah-Hartman wrote:
> > > On Tue, Nov 12, 2019 at 09:28:57AM -0600, Bin Liu wrote:
> > > > Hi,
> > > > 
> > > > On Tue, Nov 12, 2019 at 02:51:51PM +0800, Chunfeng Yun wrote:
> > > > > Now the USB gadget subsystem can use the USB debugfs root directory,
> > > > > so move musb's directory from the root of the debugfs filesystem into
> > > > > the root of usb
> > > > 
> > > > My opinion is this move is unnecessary. I breaks existing debug tools or
> > > > documentation which is already published on Internet. 
> > > 
> > > Having a "root" directory for a single random driver seems like you are
> > > making your driver a "very important" thing in the overall scheme of the
> > > kernel, right?  What's wrong with using the usb subdirectory like all
> > 
> > Agree, it wasn't the right thing to do at the first place. But now
> > changing it adds support burden, because people very often refer to the
> > old information on the internet which no longer matches to the new
> > location. Basically, it is a cost of ABI change.
> 
> What information says that /sys/kernel/debug/mdev/ is the location for
> this?  Is it in-kernel?

No, they are not in-kernel, but many places mainly on https://e2e.ti.com
and http://processors.wiki.ti.com. It basically says to get musb
regdump, cat /sys/kernel/debug/musb-hdrc.{0,1}/regdump, or to enter test
mode, do echo _testmode_ > /sys/kernel/debug/musb-hdrc.{0,1}/testmode...

> > > other USB drivers use (after this patch series is merged)?  That feels
> > > like a much more "sane" way to handle the wide-open debugfs namespace.
> > 
> > Though I commented on this musb patch, my opinion is for this whole
> > series, either drop the whole series or apply the whole series.
> 
> I've applied all but this one and 2 others that did not build properly.

Okay.

> > > Yes, there are no rules when it comes to debugfs file names and
> > > locations, but let's try to be sane please.
> > 
> > Fine with me. I can still support questions such as "can't open
> > /sys/kernel/debug/musb-hdrc.0/testmode: No such file or directory".
> 
> What tool looks for that?

I wrote a usb diagnosis tool called chkusb.sh, which basically checks
usb related information in
  - /proc/config.gz
  - /lib/modules/`uname -r`/kernel/drivers/
  - /proc/device-tree/
  - /sys/device/
  - /sys/kernel/debug/

So whenever I got a report on e2e.ti.com saying that usb doesn't work, I
just post this script and get a log which would give a clue if there was
any fundamental mistake in kernel configuration or deployment.

But not a big issue here for this tool regarding this patch set, I can
update the script to check both locations - /sys/kernel/debug/ and
/sys/kernel/debug/usb/.

-Bin.