linux-kernel - Re: [PATCH v14 8/8] soc: amd: Add support for AMD Pensando SoC Controller

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Date:   Mon, 7 Aug 2023 15:17:54 -0700
From:   Brad Larson <blarson@....com>
To:     <arnd@...db.de>
CC:     <adrian.hunter@...el.com>, <alcooperx@...il.com>,
        <andy.shevchenko@...il.com>, <blarson@....com>,
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Subject: Re: [PATCH v14 8/8] soc: amd: Add support for AMD Pensando SoC Controller

Hi Arnd,

> On Wed, May 24, 2023, at 00:11, Brad Larson wrote:
>>> On Mon, May 15, 2023, at 20:16, Brad Larson wrote:
...
>>> Also, can you explain why this needs a low-lever user interface
>>> in the first place, rather than something that can be expressed
>>> using high-level abstractions as you already do with the reset
>>> control?
>>>
>>> All of the above should be part of the changelog text to get a
>>> driver like this merged. I don't think we can get a quick
>>> solution here though, so maybe you can start by removing the
>>> ioctl side and having the rest of the driver in drivers/reset?

Might be best to pull the whole thing for now until an acceptable 
solution is reached.  The reset function is a recovery mechanisim rarely
used where the byte access to the different IP at the 4 chip-selects
is needed for a system to boot.

>> In the original patchset I added a pensando compatible to spidev and that
>> was nacked in review and reusing some random compatible that made it into 
>> spidev was just wrong.  Further it was recommended this should be a system 
>> specific driver and don't rely on a debug driver like spidev.  I changed 
>> over to a platform specific driver and at that time I also needed to include 
>> a reset controller (emmc reset only).  I put these in drivers/mfd and 
>> drivers/reset.  Review of the device tree for this approach went back and 
>> forth to _not_ have four child nodes on the spi device each with the same 
>> compatible. Decision was to squash the child nodes into the parent and put 
>> the reset-controller there also.  One driver and since its pensando
>> specific its currently in drivers/soc/amd.
>>
>> There are five different user processes and some utilities that access the 
>> functionality in the cpld/fpga.  You're correct, its passing messages that 
>> are specific to the IP accessed via chip-select.  No Elba system will boot 
>> without this driver providing ioctl access.

> Thank you for the detailed summary. Moving away from spidev and
> from mfd seems all reasonable here. I'm still a bit confused by
> why you have multiple chipselects here that are for different
> subdevices but ended with a single user interface for all of them,
> but that's not a big deal.

The goal is to isolate the the kernel from device and platform specific
changes.  All the IO to the spi connected CPLD/FPGA (design/cost dependent)
is a byte at a time or up to 16 bytes for internal flash mgmt.  Performance
is not an issue and spidev was sufficient.

Maybe this paints the right picture to zero in on a correct approach.
Internal and external IP can be present at CS1/CS2 depending on the design
where the CS0 board controller registers get additions over time in a
backward compatible manner.

Design 1: FPGA 
CS0: Board controller registers
CS1: Designware SPI to I2C to board peripherals
CS2: Lattice dual I2C master
CS3: Internal storage

Design 2: CPLD
CS0: Board controller registers
CS1: Not used or some other board specific registers
CS2: Lattice dual I2C master
CS3: Internal storage

> The main bit that sticks out about this high-level design is how
> it relies on user space utilities at all to understand the message
> format. From what I understand about the actual functionality of
> this device, it most closely resembles an embedded controller that
> you might find in a laptop or server machine, and those usually
> have kernel drivers in drivers/platform/ to interact with the
> device.

The dozens of registers at CS0 for board management are defined in
userspace programs or script.  Only the regsiter offset/bit for
emmc reset is needed for the reset function in the patches.

> Has anyone tried to do it like that? Maybe it would help
> to see what the full protocol and the user space side looks
> like, in order to move some or all of it into the kernel.

Looking at drivers/platform its pretty sparse.  What do you 
recommend based on the design 1/2 variations?

Regards,
Brad