Re: [PATCH v4 1/2] dt-bindings: usb: Add binding for discrete onboard USB hubs
From: Matthias Kaehlcke <mka@chromium.org>
Date: 2020-10-05 16:07:01
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linux-devicetree, lkml
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:quoted
On Fri, Oct 2, 2020 at 1:36 PM Alan Stern [off-list ref] wrote:quoted
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.