RE: Add stacked and parallel memories support in spi-nor

["Mahapatra, Amit Kumar" <amit.kumar-mahapatra@xxxxxxx>]

Hello Miquel,

> > - The stacked-memories DT bindings will contain the phandles of the flash nodes
> connected in stacked mode.
> >
> > - The first flash node will contain the mtd partition that would have
> > the cross over memory staring at a memory location in the first flash
> > and ending at some memory location of the 2nd flash
> 
> I don't like that much. Describing partitions past the actual device sounds wrong. If
> you look into [1] there is a suggestion from Rob to handle this case using a property
> that tells us there is a continuation, so from a software perspective we can easily
> make the link, but on the hardware description side the information are correct.

I reviewed Rob's suggestions in [1], and I need to examine the MTD layer 
to determine how this can be implemented from a software perspective. 
For reference, here is Rob's suggestion:

Describe each device and partition separately and add link(s) from one 
partition to the next 

flash0 {
  partitions {
    compatible = "fixed-partitions";
    concat-partition = <&flash1_partitions>;
    ...
  };
};

flash1 {
  flash1_partition: partitions {
    compatible = "fixed-partitions";
    ...
  };
};

> 
> If this description is accepted, then fine, you can deprecate the "stacked-memories"
> property.

I believe that in addition to Rob's description, we should also include 
the 'stacked-memories' property. This property helps us identify which 
flashes are stacked, while Rob's suggestion explains how the partitions 
within the stacked flashes are connected.

For example, if we have three flashes connected to an SPI host, with 
flash@0 and flash@1 operating in stacked mode and flash@2 functioning as a 
standalone flash, the Device Tree binding might look something like this: 
Please share your thoughts on this.

spi@0 {
  ...
  flash@0 {
    compatible = "jedec,spi-nor"
    reg = <0x00>;
    stacked-memories = <&flash@0 &flash@1>;
    spi-max-frequency = <50000000>;
    ...
        flash0_partition: partitions {
            compatible = "fixed-partitions";
	concat-partition = <&flash1_partitions>;	
        	partition@0 {
          	    label = "Stacked-Flash-1";
                reg = <0x0 0x800000>;
	}
        }
    }
  flash@1 {
    compatible = "jedec,spi-nor"
     reg = <0x01>;
    spi-max-frequency = <50000000>;
    ...
        flash1_partition: partitions {
            compatible = "fixed-partitions";
	concat-partition = <&flash0_partitions>;	
        	partition@0 {
          	    label = " Stacked-Flash-2";
                reg = <0x0 0x800000>;
	}
        }
  }

  flash@2 {
    compatible = "jedec,spi-nor"
     reg = <0x01>;
    spi-max-frequency = <50000000>;
    ...
        partitions {
            compatible = "fixed-partitions";
	concat-partition = <&flash0_partitions>;	
        	partition@0 {
          	    label = "Single-Flash";
                reg = <0x0 0x800000>;
	}
        }
  }

> 
> >  - The new layer will update the mtd->size and other mtd_info parameters after
> both the flashes are probed and will call mtd_device_register with the combined
> information.
> 
> Okay, this is back to the mtd-concat thing I initially proposed, but I believe it can
> work.

I was considering handling this by only adding a new layer between the MTD 
layer and SPI NOR without modifying the MTD layer. However, with Rob's 
suggestion, we would also need to update the mtd-concat in addition to 
adding the new layer.

> 
> [...]
> 
> > parallel-memories DT changes:
> >
> > - Flash size information can be retrieved directly from the flash, so it has been
> removed from the DT binding.
> 
> It's not really about the size but more about the fact that two memories are in use. If
> the stacked situation does not require anything specific besides the partitions trick,
> then you can assume that double reg flashes are just two flashes and this can be
> your way to discriminate the data organization. But I don't like much this shortcut
> because it is not future proof, and instead I'd keep the stacked-memory property. If

I agree, as explained above, I believe we will need the 'stacked-memories' 
property.

> you don't like the size, I don't really care, just use it as a boolean. But I believe we
> need some naming to tell the OS that the data is spread in a specific way inside the

Agree.


Regards,
Amit

> memory devices.
> 
> > - Each flash connected in parallel mode should be identical and will have one flash
> node for both the flash devices.
> 
> This is already the case.
> 
> > - The "reg" prop will contain the physical CS number for both the connected
> flashes.
> 
> This is already the case.
> 
> > - The new layer will double the mtd-> size and register it with the mtd layer.
> 
> This is not a DT change.
> 
> 
> Thanks,
> Miquèl