Re: [PATCH 1/2] hwrng: add driver for MediaTek TRNG SMC

[Daniel Golle <daniel@makrotopia.org>]

On Mon, Feb 20, 2023 at 05:58:11PM -0600, Rob Herring wrote:
> On Thu, Feb 16, 2023 at 12:32:10PM +0100, Matthias Brugger wrote:
> > 
> > 
> > On 16/02/2023 11:03, AngeloGioacchino Del Regno wrote:
> > > Il 15/02/23 14:27, Daniel Golle ha scritto:
> > > > Add driver providing kernel-side support for the Random Number
> > > > Generator hardware found on Mediatek SoCs which have a driver in ARM
> > > > TrustedFirmware-A allowing Linux to read random numbers using a
> > > > non-standard vendor-defined Secure Monitor Call.
> > > > 
> > > > Signed-off-by: Daniel Golle <daniel@makrotopia.org>
> > > 
> > > Hello Daniel,
> > > 
> > > incidentally, I've also done some research on this one some months ago, when
> > > I was deep in adding support for the Helio X10 SoC (MT6795) on Xperia M5.
> > > 
> > > The rng-v2 is simply the same rng but hypervised by the TF-A... and the only
> > > difference is, well, as you're also pointing out, that we're using secure
> > > monitor calls instead of direct MMIO handling.
> > > 
> > > There's also not much more than what you've implemented here and the only kind
> > > of addition that we will ever see on this one will be about changing the SIP
> > > command (as some older SoCs use a different one)... so...
> > > 
> > > ...I don't think that adding an entirely new driver is worth the noise, hence
> > > I propose to simply add handling for the Secure RNG to mtk-rng.c instead: it's
> > > shorter and we would only need to address one if branch on that probe function
> > > to set a different callback.
> > > 
> > > The clock should then be optional for *some* of those "v2 handling" devices,
> > > as if I recall correctly, some do need the clock to be handled from Linux
> > > anyway... otherwise this v2 driver will be "soon" looking bloody similar to
> > > the "v1", adding a bit of code duplication around.
> > > 
> > > What do you think?
> > > 
> > 
> > That was exactly what I was thinking as well when I had a look at the
> > driver. I propose to add it to mtk-rng.c. I don't see any value having a
> > second driver for this.
> 
> Or fix the firmware to use the already defined SMC TRNG interface...

I agree that this would obviously be the best solution of all, and it's
also not completely impossible as TF-A for this platform can quite easily
be built from source. However, for devices already out there it may still
not be an option.

> 
> In any case, like the SMC TRNG, you don't need a DT binding. The 
> firmware interface is discoverable. Try the SMC call and if it succeeds, 
> you have a TRNG.

I'll try that and let you know how it goes ;)


Cheers


Daniel

> 
> Rob
> 
> > 
> > Regards,
> > Matthias
> > 
> > > Regards,
> > > Angelo
> > > 
> > > > ---
> > > >   MAINTAINERS                         |  1 +
> > > >   drivers/char/hw_random/Kconfig      | 16 +++++++
> > > >   drivers/char/hw_random/Makefile     |  1 +
> > > >   drivers/char/hw_random/mtk-rng-v2.c | 74 +++++++++++++++++++++++++++++
> > > >   4 files changed, 92 insertions(+)
> > > >   create mode 100644 drivers/char/hw_random/mtk-rng-v2.c
> > > > 
> > >