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Date:   Mon, 27 Aug 2018 16:01:31 +0200
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>,
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Subject: Re: [PATCH v2 01/29] nvmem: add support for cell lookups
Message-ID: <20180827160131.58143824@bbrezillon>
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On Mon, 27 Aug 2018 15:37:23 +0200
Bartosz Golaszewski <brgl@bgdev.pl> wrote:

> 2018-08-27 11:00 GMT+02:00 Boris Brezillon <boris.brezillon@bootlin.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  
> 
> I didn't notice it before but there's a global list of nvmem cells
> with each cell referencing its owner nvmem device. I'm wondering if
> this isn't some kind of inversion of ownership. Shouldn't each nvmem
> device have a separate list of nvmem cells owned by it? What happens
> if we have two nvmem providers with the same names for cells? I'm
> asking because dev_id based lookup doesn't make sense if internally
> nvmem_cell_get_from_list() doesn't care about any device names (takes
> only the cell_id as argument).
> 
> This doesn't cause any trouble now since there are no users defining
> cells in nvmem_config - there are only DT users - but this must be
> clarified before I can advance with correctly implementing nvmem
> lookups.
> 
> BTW: of_nvmem_cell_get() seems to always allocate an nvmem_cell
> instance even if the cell for this node was already added to the nvmem
> device.

Yep, don't know if it's done on purpose or not, but it's weird. I'd
expect cells to be instantiated at NVMEM registration time (and stored
in a list attached to the device) and then, anytime someone calls
nvmem_cell_get(), you would search in this list for a match.