Re: [PATCH v5 7/8] dt-binding: mtd: Document gpio-addr-flash

[Boris Brezillon <boris.brezillon@xxxxxxxxxxx>]

On Wed, 3 Oct 2018 23:53:27 +0200
Ricardo Ribalda Delgado <ricardo.ribalda@xxxxxxxxx> wrote:

> Hi Boris
> On Wed, Oct 3, 2018 at 11:27 PM Boris Brezillon
> <boris.brezillon@xxxxxxxxxxx> wrote:
> >
> > On Wed,  3 Oct 2018 21:38:58 +0200
> > Ricardo Ribalda Delgado <ricardo.ribalda@xxxxxxxxx> wrote:
> >  
> > > Add documentation for gpio-addr-flash. This binding allow creating
> > > flash devices that are paged using GPIOs.
> > >
> > > Cc: devicetree@xxxxxxxxxxxxxxx
> > > Reviewed-by: Rob Herring <robh@xxxxxxxxxx>
> > > Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@xxxxxxxxx>
> > > ---
> > >  .../bindings/mtd/gpio-addr-flash.txt          | 54 +++++++++++++++++++
> > >  1 file changed, 54 insertions(+)
> > >  create mode 100644 Documentation/devicetree/bindings/mtd/gpio-addr-flash.txt
> > >
> > > diff --git a/Documentation/devicetree/bindings/mtd/gpio-addr-flash.txt b/Documentation/devicetree/bindings/mtd/gpio-addr-flash.txt
> > > new file mode 100644
> > > index 000000000000..5006a26e1753
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/mtd/gpio-addr-flash.txt
> > > @@ -0,0 +1,54 @@
> > > +Memory Mapped flash with some address lines addressed using GPIOs
> > > +
> > > +Handle the case where a flash device is mostly addressed using physical
> > > +line and supplemented by GPIOs.  This way you can hook up say a 8MiB flash
> > > +to a 2MiB memory range and use the GPIOs to select a particular range.
> > > +
> > > + - compatible : "cfi-gpio-addr-flash"
> > > + - reg : Address range of the mtd chip that is memory mapped, this is,
> > > +   on the previous example 2MiB.
> > > + - bank-width : Width (in bytes) of the bank.  Equal to the
> > > +   device width times the number of interleaved chips.
> > > + - gpios: List of GPIO specifiers that will be used to address the MSBs address
> > > +   lines. The order goes from LSB to MSB.
> > > + - probe-type : (optional) "cfi_probe", "jedec_probe". How the mtd chip
> > > +   is going to be probed. If omitted, assumed to be equal to "cfi_probe".  
> >
> > Looks like other bindings are encoding the probe type in the
> > compatible [1][2], and we should probably follow what's been done by
> > others.  
> 
> If I understood it right, they are special cases of physmap_of_core.c
> https://elixir.bootlin.com/linux/v4.18.11/source/drivers/mtd/maps/physmap_of_core.c#L242
> 
> The driver that handles the compatible is physmap_of_core.c, and afaik
> multiple drivers with the same
> compatible string is a very bad idea.

Yes, I know.

> 
> We can convert the driver to something like Versatile or gemini, but
> then we will not support platform devices
> 
> btw, the binding that I am used is used by:
> https://elixir.bootlin.com/linux/v4.18.11/source/drivers/mtd/maps/physmap_of_core.c#L91

Actually, the more I think about it the more I realize this should
somehow be integrated to the physmap core logic. I mean, there's nothing
controller/platform specific in what the gpio-physmap driver does.

We could basically add the msb_addr_gpios related fields to map_info,
let physmap.c and physmap_of_core.c call
devm_gpiod_get_array_optional() and, based on the returned value,
call simple_map_init() (when the pointer is NULL) or
gpio_addr_map_init() (when the pointer is valid).

gpio_addr_map_init() would still be implemented in gpio-addr-flash.c so
that we can still enable/disable support for this feature (providing
dummy wrappers when it's disabled). By doing that, we also keep a single
driver which matches a generic compatible string. We also stay
compatible with .c based board files.

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