linux-kernel - Re: [PATCH v10 1/3] nvmem: core: Rework layouts to become platform devices

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Date:   Mon, 2 Oct 2023 17:16:12 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Michael Walle <michael@...le.cc>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Robert Marko <robert.marko@...tura.hr>,
        Luka Perkov <luka.perkov@...tura.hr>,
        linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        Chen-Yu Tsai <wenst@...omium.org>,
        Daniel Golle <daniel@...rotopia.org>,
        Rafał Miłecki <rafal@...ecki.pl>
Subject: Re: [PATCH v10 1/3] nvmem: core: Rework layouts to become platform
 devices

Hi Greg,

gregkh@...uxfoundation.org wrote on Mon, 2 Oct 2023 11:35:02 +0200:

> On Fri, Sep 22, 2023 at 07:48:52PM +0200, Miquel Raynal wrote:
> > Current layout support was initially written without modules support in
> > mind. When the requirement for module support rose, the existing base
> > was improved to adopt modularization support, but kind of a design flaw
> > was introduced. With the existing implementation, when a storage device
> > registers into NVMEM, the core tries to hook a layout (if any) and
> > populates its cells immediately. This means, if the hardware description
> > expects a layout to be hooked up, but no driver was provided for that,
> > the storage medium will fail to probe and try later from
> > scratch. Technically, the layouts are more like a "plus" and, even we
> > consider that the hardware description shall be correct, we could still
> > probe the storage device (especially if it contains the rootfs).
> > 
> > One way to overcome this situation is to consider the layouts as
> > devices, and leverage the existing notifier mechanism. When a new NVMEM
> > device is registered, we can:
> > - populate its nvmem-layout child, if any
> > - try to modprobe the relevant driver, if relevant
> > - try to hook the NVMEM device with a layout in the notifier
> > And when a new layout is registered:
> > - try to hook all the existing NVMEM devices which are not yet hooked to
> >   a layout with the new layout
> > This way, there is no strong order to enforce, any NVMEM device creation
> > or NVMEM layout driver insertion will be observed as a new event which
> > may lead to the creation of additional cells, without disturbing the
> > probes with costly (and sometimes endless) deferrals.
> > 
> > Signed-off-by: Miquel Raynal <miquel.raynal@...tlin.com>  
> 
> Did I miss why these were decided to be platform devices and not normal
> devices on their own "bus" that are attached to the parent device
> properly?  Why platform for a dynamic thing?

I don't think you missed anything, following the discussion "how to
picture these layouts as devices" I came up with the simplest
approach: using the platform infrastructure. I thought creating my own
additional bus just for that would involve too much code duplication.
I agree the current implementation kind of abuses the platform
infrastructure. I will have a look in order to maybe mutate this into
its own bus.

> If I did agree with this, it should be documented here in the changelog
> why this is required to be this way so I don't ask the question again in
> the future :)

Haha, I don't think you did ;)

Thanks,
Miquèl