linux-kernel - Re: [PATCH v2] mfd: core: Support auxiliary device

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Message-ID: <Z_TYq5J0CPFvdm7e@black.fi.intel.com>
Date: Tue, 8 Apr 2025 11:04:59 +0300
From: Raag Jadav <raag.jadav@...el.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: gregkh@...uxfoundation.org, david.m.ertman@...el.com,
	ira.weiny@...el.com, lee@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mfd: core: Support auxiliary device

On Mon, Apr 07, 2025 at 11:45:30AM +0300, Andy Shevchenko wrote:
> On Mon, Apr 07, 2025 at 11:44:50AM +0300, Andy Shevchenko wrote:
> > On Mon, Apr 07, 2025 at 01:16:14PM +0530, Raag Jadav wrote:
> > > Extend MFD subsystem to support auxiliary child device. This is useful
> > > for MFD usecases where parent device is on a discoverable bus and doesn't
> > > fit into the platform device criteria. Purpose of this implementation is
> > > to provide discoverable MFDs just enough infrastructure to register
> > > independent child devices with their own memory and interrupt resources
> > > without abusing the platform device.
> > > 
> > > Current support is limited to just PCI type MFDs, but this can be further
> > > extended to support other types like USB in the future.
> > 
> > > PS: I'm leaning towards not doing any of the ioremap or regmap on MFD
> > > side and think that we should enforce child devices to not overlap.
> > 
> > Yes, but we will have the cases in the future, whatever,
> > for the first step it's okay.
> > 
> > > If there's a need to handle common register access by parent device,
> > > then I think it warrants its own driver which adds auxiliary devices
> > > along with a custom interface to communicate with them, and MFD on
> > > AUX is not the right solution for it.
> 
> And yes, I still consider enforcing regmap is the right step to go.

Unless there's an explicit need for it, I think we should leave that
choice to the individial drivers instead of forcing them to revamp.
But let's see what Lee and Greg have to say about this.

Raag