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From:   Bartosz Golaszewski <brgl@bgdev.pl>
Date:   Mon, 27 Aug 2018 10:56:29 +0200
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Subject: Re: [PATCH v2 01/29] nvmem: add support for cell lookups
To:     Boris Brezillon <boris.brezillon@bootlin.com>
Cc:     Andrew Lunn <andrew@lunn.ch>,
        linux-doc <linux-doc@vger.kernel.org>,
        Sekhar Nori <nsekhar@ti.com>,
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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.

> 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?

Best regards,
Bart