Re: [PATCH v5 3/3] arm: dts: am335x-sancloud-bbe-lite: UEFI SPI export

[Paul Barker <paul.barker@sancloud.com>]

On 20/12/2022 15:55, Rob Herring wrote:
> On Wed, Nov 23, 2022 at 05:50:06PM +0000, Paul Barker wrote:
>> Add properties to the Authenta SPI flash device node to enable access by
>> a UEFI application using a fixed GUID.
>>
>> Signed-off-by: Paul Barker <paul.barker@sancloud.com>
>> ---
>>  arch/arm/dts/am335x-sancloud-bbe-lite-u-boot.dtsi | 13 ++++++++++---
>>  arch/arm/dts/am335x-sancloud-bbe-lite.dts         |  2 +-
>>  2 files changed, 11 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/arm/dts/am335x-sancloud-bbe-lite-u-boot.dtsi b/arch/arm/dts/am335x-sancloud-bbe-lite-u-boot.dtsi
>> index 01c105ebb383..6c4ff67f9a4b 100644
>> --- a/arch/arm/dts/am335x-sancloud-bbe-lite-u-boot.dtsi
>> +++ b/arch/arm/dts/am335x-sancloud-bbe-lite-u-boot.dtsi
>> @@ -38,7 +38,14 @@
>>  
>>  &spi0 {
>>  	u-boot,dm-pre-reloc;
>> -	channel@0 {
>> -		u-boot,dm-pre-reloc;
>> -	};
>> +};
>> +
>> +&authenta_flash {
>> +	u-boot,dm-pre-reloc;
>> +
>> +	u-boot,uefi-spi-vendor = "micron";
>> +	u-boot,uefi-spi-part-number = "mt25ql128abb";
> 
> Looks like a compatible string. Yet, the flash node compatible string, 
> micron,spi-authenta, is not documented (though in use for spidev). So 
> use a compatible string for the flash that is specific to the flash 
> model. I assume there is some reason the specific model is needed?

For context, the UEFI Platform Initialization (PI) spec defines
EFI_SPI_PART, EFI_SPI_PERIPHERAL and EFI_SPI_IO_PROTOCOL structures.
I'm referencing v1.7 Errata A. See https://uefi.org/specifications for
downloads.

The EFI_SPI_PART structure has "Vendor" and "PartNumber" fields. We need
something to put in those fields and the device tree is the best place
to store the data. These properties are in the `-u-boot.dtsi` file so
they won't be submitted to the Linux kernel.

The `micron,spi-authenta` compatible string is unfortunate in my
opinion. I plan to replace this with two entries, `micron,mt25ql128abb`
and `jedec,spi-nor`, once we've got MTD support for the Authenta parts
merged into the mainline kernel. That work is currently on hold, I'll
be working with Micron in the new year to unblock it. We could submit
a change to the compatible property in the meantime if you think it's
worth getting a temporary improvement in before the MTD changes are
done.

>> +	/* GUID in UEFI format: 77126730-a4ca-4386-b341-881fe18e7f7d */
>> +	u-boot,uefi-spi-io-guid = [30 67 12 77 ca a4 86 43
>> +				   b3 41 88 1f e1 8e 7f 7d];
> 
> We need to define first in the DT spec the format for GUIDs. I don't 
> think there are any existing cases though some have been proposed. IMO, 
> I would make this a string instead. The byte array is not that 
> readable with its little endian order. A GUID as a string is readily 
> identifiable as a GUID.

I see there is a `uuid_str_to_bin()` function in u-boot so I could make
this a string property and convert it to a binary GUID at runtime. This
would make the dts more readable so I'll make the change for v6 of this
series.

> Why is this u-boot specific? Another UEFI implementation doesn't need 
> the GUID?

Each EFI_SPI_IO_PROTOCOL instance needs its own GUID so that it can
be located at runtime. These GUIDs are not pre-defined by the spec,
instead an arbitrary per-device GUID is used and is stored in the
SpiPeripheralDriverGuid field of the relevant EFI_SPI_PERIPHERAL
instance. We could randomly generate these GUIDs at runtime since a UEFI
application can walk the tree of SPI busses and peripherals, looking
for a match in the Vendor and PartNumber fields defined above, to get
the appropriate GUID. However, storing a known value in the device tree
allows a UEFI application to just lookup the GUID without needing to
walk the SPI tree.

I'm happy to consider alternatives to the current implementation,
however we need to work within the limits of the UEFI PI spec.

Thanks,

-- 
Paul Barker
Principal Software Engineer
SanCloud Ltd

e: paul.barker@sancloud.com
w: https://sancloud.com/