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From: Matthias Kaehlcke <mka@chromium.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Rob Herring <robh@kernel.org>,
	Doug Anderson <dianders@chromium.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Frank Rowand <frowand.list@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Linux USB List <linux-usb@vger.kernel.org>,
	Bastien Nocera <hadess@hadess.net>,
	Stephen Boyd <swboyd@chromium.org>,
	Ravi Chandra Sadineni <ravisadineni@chromium.org>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@vger.kernel.org>, Peter Chen <peter.chen@nxp.com>
Subject: Re: [PATCH v4 1/2] dt-bindings: usb: Add binding for discrete onboard USB hubs
Date: Mon, 5 Oct 2020 09:06:55 -0700	[thread overview]
Message-ID: <20201005160655.GA4135817@google.com> (raw)
In-Reply-To: <20201003124142.GA318272@rowland.harvard.edu>

On Sat, Oct 03, 2020 at 08:41:42AM -0400, Alan Stern wrote:
> On Fri, Oct 02, 2020 at 05:58:22PM -0500, Rob Herring wrote:
> > On Fri, Oct 2, 2020 at 1:36 PM Alan Stern <stern@rowland.harvard.edu> wrote:
> > > Regardless of how the situation is represented in DT, there remains the
> > > issue of where (i.e., in which driver module) the appropriate code
> > > belongs.  This goes far beyond USB.  In general, what happens when one
> > > sort of device normally isn't hooked up through a power regulator, so
> > > its driver doesn't have any code to enable a regulator, but then some
> > > system does exactly that?
> > >
> > > Even worse, what if the device is on a discoverable bus, so the driver
> > > doesn't get invoked at all until the device is discovered, but on the
> > > new system it can't be discovered until the regulator is enabled?
> > 
> > Yep, it's the same issue here with USB, MDIO which just came up a few
> > weeks ago, MMC/SD which hacked around it with 'mmc-pwrseq' binding
> > (not something I want to duplicate) and every other discoverable bus.
> > What do they all have in common? The kernel's driver model being
> > unable to cope with this situation. We really need a common solution
> > here and not bus or device specific hack-arounds.
> 
> To me this doesn't seem quite so much to be a weakness of the kernel's 
> driver model.
> 
> It's a platform-specific property, one that is not discoverable and 
> therefore needs to be represented somehow in DT or ACPI or something 
> similar.  Something that says "Device A cannot operate or be discovered 
> until power regulator B is enabled", for example.
> 
> The decision to enable the power regulator at system startup would be 
> kernel policy, not a part of the DT description.  But there ought to be 
> a standard way of recognizing which resource requirements of this sort 
> should be handled at startup.  Then there could be a special module (in 
> the driver model core? -- that doesn't really seem appropriate) which 
> would search through the whole DT database for resources of this kind 
> and enable them.

This might work for some cases that only have a single resource or multiple
resources but no timing/sequencing requirements. For the more complex cases
it would probably end up in something similar to the pwrseq series
(https://lore.kernel.org/patchwork/project/lkml/list/?series=314989&state=%2A&archive=both),
which was nack-ed by Rafael, Rob also expressed he didn't want to go
down that road.

It seems to me that initialization of the resources needs to be done by
the/a driver for the device, which knows about the sequencing requirements.
Potentially this could be done in a pre-probe function that you brought up
earlier.

  reply	other threads:[~2020-10-05 16:06 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-28 17:13 [PATCH v4 1/2] dt-bindings: usb: Add binding for discrete onboard USB hubs Matthias Kaehlcke
2020-09-28 17:13 ` [PATCH v4 2/2] USB: misc: Add onboard_usb_hub driver Matthias Kaehlcke
2020-09-28 18:47   ` Alan Stern
2020-09-29  1:43     ` Matthias Kaehlcke
2020-09-29 16:00       ` Alan Stern
2020-09-29 16:50         ` Matthias Kaehlcke
2020-09-28 22:03   ` Doug Anderson
2020-09-29  1:59     ` Matthias Kaehlcke
2020-09-28 22:13 ` [PATCH v4 1/2] dt-bindings: usb: Add binding for discrete onboard USB hubs Doug Anderson
2020-09-29  2:14   ` Matthias Kaehlcke
2020-09-29 20:17 ` Rob Herring
2020-09-29 22:09   ` Matthias Kaehlcke
2020-09-30  1:32     ` Alan Stern
2020-09-30 12:49       ` Matthias Kaehlcke
2020-09-30 14:44         ` Rob Herring
2020-09-30 15:28           ` Doug Anderson
2020-09-30 18:00             ` Doug Anderson
2020-09-30 19:19               ` Rob Herring
2020-10-01 21:39                 ` Matthias Kaehlcke
2020-09-30 19:19             ` Alan Stern
2020-09-30 20:20             ` Rob Herring
2020-10-01  1:24               ` Alan Stern
2020-10-01 21:54                 ` Matthias Kaehlcke
2020-10-02  1:21                   ` Alan Stern
2020-10-02 16:08                     ` Matthias Kaehlcke
2020-10-02 18:48                       ` Alan Stern
2020-10-02 17:08               ` Doug Anderson
2020-10-02 18:36                 ` Alan Stern
2020-10-02 22:58                   ` Rob Herring
2020-10-03 12:41                     ` Alan Stern
2020-10-05 16:06                       ` Matthias Kaehlcke [this message]
2020-10-05 16:15                         ` Alan Stern
2020-10-05 19:18                           ` Matthias Kaehlcke
2020-10-05 19:36                             ` Alan Stern
2020-10-05 19:59                               ` Rob Herring
2020-10-05 23:29                                 ` Matthias Kaehlcke
2020-10-05 19:36                             ` Rob Herring
2020-10-05 19:20                         ` Rob Herring
2020-10-02 22:28                 ` Rob Herring
2020-10-02 23:09                   ` Doug Anderson
2020-10-06  0:45                     ` Matthias Kaehlcke
2020-10-06 14:18                       ` Alan Stern
2020-10-06 16:59                         ` Matthias Kaehlcke
2020-10-06 17:15                           ` Alan Stern
2020-10-06 19:25                             ` Matthias Kaehlcke
2020-10-07  1:00                               ` Alan Stern
2020-10-07 16:03                                 ` Matthias Kaehlcke
2020-10-07 16:38                                   ` Alan Stern
2020-10-07 17:28                                     ` Matthias Kaehlcke
2020-10-07 19:25                                       ` Alan Stern
2020-10-07 19:42                                         ` Matthias Kaehlcke
2020-10-07 20:17                                           ` Alan Stern
2020-10-07 21:42                                             ` Matthias Kaehlcke
2020-10-08 14:09                                               ` Alan Stern
2020-10-09 23:13                                                 ` Matthias Kaehlcke

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