linux-kernel - Re: [RFC 0/2] i2c-designware: Add support for AMD PSP semaphore

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Date:   Wed, 22 Dec 2021 20:21:15 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     Jan Dabros <jsd@...ihalf.com>, linux-kernel@...r.kernel.org,
        linux-i2c@...r.kernel.org, jarkko.nikula@...ux.intel.com,
        mika.westerberg@...ux.intel.com, wsa@...nel.org,
        rrangel@...omium.org, mw@...ihalf.com, jaz@...ihalf.com,
        upstream@...ihalf.com
Subject: Re: [RFC 0/2] i2c-designware: Add support for AMD PSP semaphore

On Wed, Dec 22, 2021 at 08:56:21PM +0300, Serge Semin wrote:
> On Wed, Dec 22, 2021 at 01:46:07PM +0200, Andy Shevchenko wrote:
> > On Wed, Dec 22, 2021 at 10:45:56AM +0100, Jan Dabros wrote:
> > > This patchset comprises support for new i2c-designware controller setup on some
> > > AMD Cezanne SoCs, where x86 is sharing i2c bus with PSP. PSP uses the same
> > > controller and acts as an i2c arbitrator there (x86 is leasing bus from it).
> > > 
> > > First commit aims to improve generic i2c-designware code by adding extra locking
> > > on probe() and disable() paths. I would like to ask someone with access to
> > > boards which use Intel BayTrail(CONFIG_I2C_DESIGNWARE_BAYTRAIL) to verify
> > > behavior of my changes on such setup.
> > > 
> > > Second commit adds support for new PSP semaphore arbitration mechanism.
> > > Implementation is similar to the one from i2c-designware-baytrail.c however
> > > there are two main differences:
> > > 1) Add new ACPI ID in order to protect against silent binding of the old driver
> > > to the setup with PSP semaphore. Extra flag ARBITRATION_SEMAPHORE added to this
> > > new _HID allows to recognize setup with PSP.
> > > 2) Beside acquire_lock() and release_lock() methods we are also applying quirks
> > > to the lock_bus() and unlock_bus() global adapter methods. With this in place
> > > all i2c clients drivers may lock i2c bus for a desired number of i2c
> > > transactions (e.g. write-wait-read) without being aware of that such bus is
> > > shared with another entity.
> > > 
> > > Mark this patchset as RFC, since waiting for new ACPI ID value. As a temporary
> > > measure use "AMDI9999". Once proper one will be ready, will re-send this CL for
> > > review & merge.
> > > 
> > > Looking forward to some feedback.
> > 
> > If I am not mistaken something similar happened in Baikal T1.
> > Perhaps Serge has something to share.
> 
> No, Baikal-T1 doesn't have such specific interface sharing since it
> doesn't have any co-processor (though a scenario of booting different
> kernels on each CPU core was at consideration by some our customers).
> So the only peculiar things the SoC has got are two interfaces with
> non-standard access:
> 1) DW SPI controller with memory mapped 16MB direct EEPROM access. DW
> SPI CSR/EEPROM mapping are switched by a multiplexer (basically just a
> flag) embedded into the system controller.
> 2) DW i2c controller with indirect registers access. Originally it was
> supposed to be used by the system bootloader for some bootloading
> stuff, but the actual usage scenario wasn't described by the SoC
> engineers. The chip initially loads the code from the SPI-flash only,
> which can be of up to 16MB size. It's more than enough to start pretty
> complex systems, so an additional i2c interface turned to be not
> really needed. Anyway other than having an indirectly accessible
> CSRs it's pretty much normal DW I2C controller.
> 
> But you are right in a reference to another BE-chip - Baikal-M1. The
> i2c/spi/gpio/uart interfaces sharing support might get to be needed
> for it since aside with four 2-cored Cortex-A57 clusters it has got an
> embedded SCP co-processor which can access the same SoC interfaces as
> the CPU cores. Though Baikal-M1 isn't supported by the mainline kernel
> at the moment. We are going to start working on it on the next year.
> Then we'll most likely need to implement the interface sharing feature
> like the one introduced in this RFC. But for now alas I can't be much
> helpful.

Anyway, thanks for your insights!

-- 
With Best Regards,
Andy Shevchenko