[U-Boot] [PATCH 05/19] riscv: Add a SYSCON driver for Core Local Interruptor

[Bin Meng]

Hi Lukas,

On Wed, Nov 14, 2018 at 6:33 PM Auer, Lukas
<lukas.auer at aisec.fraunhofer.de> wrote:
>
> Hi Bin,
>
> On Wed, 2018-11-14 at 09:48 +0800, Bin Meng wrote:
> > Hi Lukas,
> >
> > On Tue, Nov 13, 2018 at 10:45 PM Auer, Lukas
> > <lukas.auer at aisec.fraunhofer.de> wrote:
> > >
> > > Hi Bin,
> > >
> > > On Tue, 2018-11-13 at 00:21 -0800, Bin Meng wrote:
> > > > This adds U-Boot syscon driver for RISC-V Core Local Interruptor
> > > > (CLINT). The CLINT block holds memory-mapped control and status
> > > > registers associated with software and timer interrupts.
> > > >
> > > > 3 APIs are provided for U-Boot to implement Supervisor Binary
> > > > Interface (SBI) as defined by the RISC-V privileged architecture
> > > > spec v1.10.
> > > >
> > > > Signed-off-by: Bin Meng <bmeng.cn at gmail.com>
> > > > ---
> > >
> > > Would it make sense to also abstract the functions provided by the
> > > CLINT more? The reason why I am asking is because the CLINT is
> > > (unfortunately) not specified as part of RISC-V. It is developing
> > > into
> > > a de facto standard since other platforms are following SiFive's
> > > implementation, but there is nothing that would prevent them from
> > > implementing something else.
> > >
> >
> > I think your observation is correct about CLINT. Rick, does Andes's
> > RISC-V processor also follow SiFive's CLINT model?
> >
> > > Two immediate examples I can think of would be mtime and the IPI
> > > implementation. Many SoC vendors will likely already have a
> > > suitable
> > > timer IP block for mtime. I can imagine that they would choose to
> > > re-
> > > use their memory map instead of following that of the CLINT.
> > > For the IPI implementation there is already an alternative, the
> > > SBI. If
> > > U-Boot should be able to run in supervisor mode, it would be
> > > helpful to
> > > support both the SBI and the CLINT interface.
> > >
> >
> > I am not sure I followed you here. This driver provides 3 APIs:
> > riscv_send_ipi() is for IPI, and the other 2 are for mtime/mtimecmp.
> >
>
> It does, but I am not sure how easy it is to support different devices.
> Supporting the SBI is not going to be an issue, more problematic would
> be if we have two different devices and device tree nodes to implement
> the functionality of the APIs. Now we have to bind this driver to two
> devices and call the APIs from the correct instantiation, which would
> not work.
>
> Thinking about this a little more, I think the only issue is that we
> have both IPI- and mtime-related APIs in one driver. How about
> something like this? Instead of binding this driver to riscv,clint0, we
> add a new misc driver for the clint0. The only thing the driver does,
> is to bind the syscon driver and the timer driver (see for example
> tegra-car.c). Other architectures with separate device tree nodes for
> the API functionality won't need the misc driver and can just bind the
> devices to the syscon driver and a timer driver.
>

Sorry it took a long time before replying this. I did have a look at
the tegra-car.c driver, and also tried various experiments. As Rick
pointed out we have to handle mixed IP blocks like Andes chip for
mtimer and IPIs. So it looks we need be able to flexibly handle the
following cases (sigh):

- SiFive's clint model which implements mtimer and IPI
- mtimer following SiFive's clint model, but IPI does not (Andes chip)
- IPI following SiFive's clint model, but mtimer follows (hypothetical
model which does not exist today)
- completely different mtimer and IPI models from SiFive's clint model

 I don't quite follow your idea of implementing clint as a misc
driver, then binding syscon and timer devices in the misc driver. But
I think we can only have the misc driver, and use misc uclass's
ioctl() to implement the SBI required calls (set_timecmp, get_time,
send_ipi), and we can have another ioctl code to report its capability
to support mtimer and/or IPI functionality. This can support
flexibility. However I believe this may affect performance if we go
through many uclass function calls when doing SBI.

The solution does not look clean to me :(

Regards,
Bin