Re: [PATCH v2 01/29] nvmem: add support for cell lookups

From: Boris Brezillon <boris.brezillon@bootlin.com>
To: Bartosz Golaszewski <brgl@bgdev.pl>
Cc: Andrew Lunn <andrew@lunn.ch>,
	linux-doc <linux-doc@vger.kernel.org>,
	Sekhar Nori <nsekhar@ti.com>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Srinivas Kandagatla <srinivas.kandagatla@linaro.org>,
	linux-i2c <linux-i2c@vger.kernel.org>,
	Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
	Rob Herring <robh@kernel.org>,
	Florian Fainelli <f.fainelli@gmail.com>,
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	Richard Weinberger <richard@nod.at>,
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Subject: Re: [PATCH v2 01/29] nvmem: add support for cell lookups
Date: Mon, 27 Aug 2018 11:00:55 +0200	[thread overview]
Message-ID: <20180827110055.122988d0@bbrezillon> (raw)
In-Reply-To: <CAMRc=Men-MPk5DGshWcVEc0v=gH2WSpx1j-CawOeydwp59tejw@mail.gmail.com>

On Mon, 27 Aug 2018 10:56:29 +0200
Bartosz Golaszewski <brgl@bgdev.pl> wrote:

> 2018-08-25 8:27 GMT+02:00 Boris Brezillon <boris.brezillon@bootlin.com>:
> > On Fri, 24 Aug 2018 17:27:40 +0200
> > Andrew Lunn <andrew@lunn.ch> wrote:
> >  
> >> On Fri, Aug 24, 2018 at 05:08:48PM +0200, Boris Brezillon wrote:  
> >> > Hi Bartosz,
> >> >
> >> > On Fri, 10 Aug 2018 10:04:58 +0200
> >> > Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> >> >  
> >> > > +struct nvmem_cell_lookup {
> >> > > + struct nvmem_cell_info  info;
> >> > > + struct list_head        list;
> >> > > + const char              *nvmem_name;
> >> > > +};  
> >> >
> >> > Hm, maybe I don't get it right, but this looks suspicious. Usually the
> >> > consumer lookup table is here to attach device specific names to
> >> > external resources.
> >> >
> >> > So what I'd expect here is:
> >> >
> >> > struct nvmem_cell_lookup {
> >> >     /* The nvmem device name. */
> >> >     const char *nvmem_name;
> >> >
> >> >     /* The nvmem cell name */
> >> >     const char *nvmem_cell_name;
> >> >
> >> >     /*
> >> >      * The local resource name. Basically what you have in the
> >> >      * nvmem-cell-names prop.
> >> >      */
> >> >     const char *conid;
> >> > };
> >> >
> >> > struct nvmem_cell_lookup_table {
> >> >     struct list_head list;
> >> >
> >> >     /* ID of the consumer device. */
> >> >     const char *devid;
> >> >
> >> >     /* Array of cell lookup entries. */
> >> >     unsigned int ncells;
> >> >     const struct nvmem_cell_lookup *cells;
> >> > };
> >> >
> >> > Looks like your nvmem_cell_lookup is more something used to attach cells
> >> > to an nvmem device, which is NVMEM provider's responsibility not the
> >> > consumer one.  
> >>
> >> Hi Boris
> >>
> >> There are cases where there is not a clear providier/consumer split. I
> >> have an x86 platform, with a few at24 EEPROMs on it. It uses an off
> >> the shelf Komtron module, placed on a custom carrier board. One of the
> >> EEPROMs contains the hardware variant information. Once i know the
> >> variant, i need to instantiate other I2C, SPI, MDIO devices, all using
> >> platform devices, since this is x86, no DT available.
> >>
> >> So the first thing my x86 platform device does is instantiate the
> >> first i2c device for the AT24. Once the EEPROM pops into existence, i
> >> need to add nvmem cells onto it. So at that point, the x86 platform
> >> driver is playing the provider role. Once the cells are added, i can
> >> then use nvmem consumer interfaces to get the contents of the cell,
> >> run a checksum, and instantiate the other devices.
> >>
> >> I wish the embedded world was all DT, but the reality is that it is
> >> not :-(  
> >
> > Actually, I'm not questioning the need for this feature (being able to
> > attach NVMEM cells to an NVMEM device on a platform that does not use
> > DT). What I'm saying is that this functionality is provider related,
> > not consumer related. Also, I wonder if defining such NVMEM cells
> > shouldn't go through the provider driver instead of being passed
> > directly to the NVMEM layer, because nvmem_config already have a fields
> > to pass cells at registration time, plus, the name of the NVMEM cell
> > device is sometimes created dynamically and can be hard to guess at
> > platform_device registration time.
> >  
> 
> In my use case the provider is at24 EEPROM driver. This is where the
> nvmem_config lives but I can't image a correct and clean way of
> passing this cell config to the driver from board files without using
> new ugly fields in platform_data which this very series is trying to
> remove. This is why this cell config should live in machine code.

Okay.

> 
> > I also think non-DT consumers will need a way to reference exiting
> > NVMEM cells, but this consumer-oriented nvmem cell lookup table should
> > look like the gpio or pwm lookup table (basically what I proposed in my
> > previous email).  
> 
> How about introducing two new interfaces to nvmem: one for defining
> nvmem cells from machine code and the second for connecting these
> cells with devices?

Yes, that's basically what I was suggesting: move what you've done in
nvmem-provider.h (maybe rename some of the structs to make it clear
that this is about defining cells not referencing existing ones), and
add a new consumer interface (based on what other subsystems do) in
nvmem-consumer.h.

This way you have both things clearly separated, and if a driver is
both a consumer and a provider you'll just have to include both headers.

Regards,

Boris