[U-Boot] Adding support for SPI-attached MR25H256 MRAM with Device Tree

[Simon Falsig]

>On 20 November 2015 at 19:13, Simon Falsig <simon at newtec.dk> wrote:
>> I have a custom board based on a TI AM3356 processor (quite similar to
>> a BeagleBone) with an Everspin MR25H256 MRAM attached to the SPI bus,
>> currently using U-Boot 2015.10 to boot a 3.18.9-rt5 Linux kernel.
>>
>> I'd like to use this MRAM to store the MAC address for one of the
>> Ethernet interfaces, but I'm having a bit of trouble getting started
>> on how support for this should be added.
>>
>> I can access the MRAM just fine from Linux (using the m25p80 driver
>> and the MTD framework), so with the appropriate drivers compiled into
>> the kernel, Linux picks the chip up without any issue by simply adding
>> the following to the device tree:
>>
>>         mram: m25p80 at 0 {
>>                 #address-cells = <1>;
>>                 #size-cells = <1>;
>>                 compatible = "mr25h256";
>>                 reg = <0>;
>>                 spi-max-frequency = <40000000>;
>>                 status = "okay";
>>         };
>>
>> I'd like U-Boot to do something similar, but there are a few issues:
>>   1. The chip is not JEDEC compatible, so it cannot be probed like
>>      the other chips currently listed in drivers/mtd/spi/sf_params.c
>>   2. Since it's an MRAM, it doesn't need to be erased before being
>>      written - Linux has a flag for this, but it doesn't seem as if
>>      U-Boot has anything similar. (Not that I need this currently
>>      though, as I only need to be able to read the chip for now - but
>>      it'd be nice to have, and others might be able to use it too.)
>>
>
>Thanks for your concerns, It's been very clear.
>
>> I've tried having a look at doc/driver-model/spi-howto.txt, hoping
>> that this would have up-to-date information on how to implement/modify
>> a SPI driver, but when I for instance look at spi_setup_slave_fdt(),
>> which seems to be one of the necessary functions, I can only find it
>> implemented in drivers/spi/spi-uclass.c, with a comment saying that it
>> is only for compatibility, and that it is to be removed?...
>
>These are spi drivers not related to spi-flash.
>drivers/mtd/spi/sf_probe.c
>drivers/mtd/spi/sf_ops.c are spi-flash related.
>
>If possible you can add non-jedec things to sf_probe.c similar way as
>Linux, BTW I'm >currently working spi-flash subsystem which should as
>modular as Linux[1]. I hope we may >get that version coming releases, but
>what ever is feasible to you just work on that I >would help you always.
>
>>
>> If anyone can help me with a few hints on how to get started adding
>> support for this chip, it'd be much appreciated!
>>
>> Thanks in advance and best regards,
>
>[1]
>http://git.denx.de/?p=u-boot/u-boot-spi.git;a=shortlog;h=refs/heads/spi-nor-mtd
>
>--
>Jagan | openedev.

Thanks for the hints! I'll have a look at the existing files and your tree,
and see if I can find a good way to do this.