SPI driver for Raspberry Pi 4 (BMC2835)

SPI driver for Raspberry Pi 4 (BMC2835)

[J. Holland]

Hi,

does anyone know, if there is a SPI driver for Raspberry Pi 4 in U-Boot?

I want to communicate with a TPM 2.0 via hardware SPI. So far I've compiled
mainline U-Boot with TPM driver support. However, the SPI driver seems to be
missing.

The linux kernel ships the driver (drivers/spi/spi-bcm2835.c). Does
anyone know
if there is there a port for U-Boot or if anyone is currently working on it?

In theory, I should be able to use the soft SPI driver (since there is a
GPIO
driver), but I'd very much prefer using the hardware SPI controller.

Many thanks!

Best,
Johannes

SPI driver for Raspberry Pi 4 (BMC2835)

[pierre fontaine]

Hi Johannes,

I'm also trying to make a secure boot using an SLB9670 TPM (SPI), Yocto
and a Raspberry 3, and as far as I'm documented
there are no hardware SPI driver due to the fact that `start.elf` and
`bootcode.bin` are being closed binaries
(https://pi3g.com/2019/02/04/qa-letstrust-tpm/).

However some people tried to deal with software SPI driver as a
workaround, but I don't know yet if it works (@see:
https://www.mail-archive.com/u-boot at lists.denx.de/msg330403.html).

Hope it brings some clues, if you manage to do something keep me updated !

Have a nice day,

Pierre Fontaine

Le 25/03/2020 ? 09:08, J. Holland a ?crit?:
> Hi,
>
> does anyone know, if there is a SPI driver for Raspberry Pi 4 in U-Boot?
>
> I want to communicate with a TPM 2.0 via hardware SPI. So far I've
> compiled
> mainline U-Boot with TPM driver support. However, the SPI driver seems
> to be
> missing.
>
> The linux kernel ships the driver (drivers/spi/spi-bcm2835.c). Does
> anyone know
> if there is there a port for U-Boot or if anyone is currently working
> on it?
>
> In theory, I should be able to use the soft SPI driver (since there is a
> GPIO
> driver), but I'd very much prefer using the hardware SPI controller.
>
> Many thanks!
>
> Best,
> Johannes
>

SPI driver for Raspberry Pi 4 (BMC2835)

[J. Holland]

Hi Pierre,

> I'm also trying to make a secure boot using an SLB9670 TPM (SPI), Yocto
> and a Raspberry 3, and as far as I'm documented
> there are no hardware SPI driver due to the fact that `start.elf` and
> `bootcode.bin` are being closed binaries
> (https://pi3g.com/2019/02/04/qa-letstrust-tpm/).

There is no driver because nobody wrote one for u-boot, yet. The linux kernel
does have the right driver, so closed source is not the problem. The thing is,
that nobody ported it to u-boot, yet.

What the Let's Trust website refers to is that *secure boot* is not possible
because there is no open-source first-level bootloader, only a closed-source
proprietary one. You can use u-boot as a second-stage bootloader and use the
TPM. It's just not secure boot since you need a so-called Root of Trust in the
first-stage bootloader, which we do not have.

> However some people tried to deal with software SPI driver as a
> workaround, but I don't know yet if it works (@see:
> https://www.mail-archive.com/u-boot at lists.denx.de/msg330403.html).
>
> Hope it brings some clues, if you manage to do something keep me updated !

Thanks for the hint. In fact, I managed to solve the issue. The problem is, that
the soft-spi driver does not implement the SPI modes properly. The Infineon TPM
slb9670 expects mode 0 (CPOL=0, CPHA=0), but the driver has CPOL=1 hardcoded and
operates on CPHA=1 by default. After fixing the issue, using the TPM works.
However, you might not be able to use the TPM in the Raspbian kernel anymore.
I'm working on that, currently.

See my patch:
https://patchwork.ozlabs.org/patch/1263827/

Thanks,
Johannes