Date:   Tue, 2 Jan 2018 23:36:52 +0800
From:   "Wang, Haiyue" <haiyue.wang@...ux.intel.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Joel Stanley <joel@....id.au>, gregkh <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        linux-spi <linux-spi@...r.kernel.org>
Subject: Re: [PATCH v1] eSPI: add Aspeed AST2500 eSPI driver to boot a host
 with PCH runs on eSPI



On 2018-01-02 23:13, Arnd Bergmann wrote:
>> On 2017-12-31 07:10, Arnd Bergmann wrote:
>>> On Fri, Dec 29, 2017 at 2:53 AM, Haiyue Wang
>>> <haiyue.wang@...ux.intel.com> wrote:
>>>> When PCH works under eSPI mode, the PMC (Power Management Controller) in
>>>> PCH is waiting for SUS_ACK from BMC after it alerts SUS_WARN. It is in
>>>> dead loop if no SUS_ACK assert. This is the basic requirement for the BMC
>>>> works as eSPI slave.
>>>>
>>>> Also for the host power on / off actions, from BMC side, the following VW
>>>> (Virtual Wire) messages are done in firmware:
>>>> 1. SLAVE_BOOT_LOAD_DONE / SLAVE_BOOT_LOAD_STATUS
>>>> 2. SUS_ACK
>>>> 3. OOB_RESET_ACK
>>>> 4. HOST_RESET_ACK
>>> I have not looked at the driver contents yet, but I'm adding the SPI
>>> maintainer and
>>> mailing list to Cc here for further discussion. Can you clarify how
>>> the eSPI slave
>>> mode relates to SPI slaves that we already support? I was under the impression
>>> that the difference between SPI and eSPI is mainly on the master side, but that
>>> any SPI slave can also act as an eSPI slave. Would this driver fit into the SPI
>>> slave framework, possibly with some extensions to the generic abstraction?
>> In simple word, the eSPI uses the SPI interface pin definition, but it
>> will replace Low Pin Count (LPC)
>> interface. From its name, sure, it will confuse you! ;-)
> I know what eSPI is meant for, and understand the basic idea of the
> protocol, but I'm not familiar with the Apeed slave hardware
> implementation.
I see! ;-)
>>> It also seems rather inflexible to have a single driver that is responsible both
>>> for the transport (eSPI register level interface for ASPEED) and the high-level
>>> protocol (talking to an Intel PCH), since either half of the work could be
>>> done elsewhere, using either a different eSPI slave implementation, or
>>> a different
>>> host architecture)
>> Yes, eSPI has the architecture such as transaction layer, link Layer;
>> all of it is about the **silicon**
>> design. That's why I put the driver under /misc directory, not /spi
>> directory.
> I don't see any requirement in the eSPI spec for the upper layers to
> be implemented in hardware. Obviously an x86 host such as Intel's
> PCH would implement the host interface using PIO,  and MMIO
> accesses that are compatible with ISA and LPC, as this is the motivation
> behind the specification, but an ARM server that wants to use eSPI
> based peripherals could choose to implement it just as well using
> a traditional SPI master hardware, some GPIOs (reset and alert)
> and a (driver independent) software implementation of the transaction
> and link layers.
>
> On the slave side, it seems that aspeed have implemented the
> virtual wires partially in hardware and require a driver like the one
> you wrote to reply to some of the wires being accessed by the host,
> but again it seems plausible that this could be implemented in another
> BMC using a generic SPI slave and a transaction layer written
> entirely in software.
Yes, you are right, Aspeed have implemented the virtual wires partially. 
Tthat's why I named it
as aspeed-espi-slave driver.
> Your driver does not handle the other channels (smbus, mmio, spinor)
> at the moment at all, can you provide some information how they
> are implemented in the ast2500? Are those handled completely
> in hardware (I assume this is the case for spinor at least), or do they
> require help from a driver, either this one or a separate one?
I can't send the AST2500 datasheet to you directly, but you can contact 
Aspeed firstly.
https://www.aspeedtech.com/products.php?fPath=20&rId=440

These functions are handled in hardware, the original SDK just provides 
some ioctl API for user
application to access them. The mmio function such as KCS / Port 80, 
these controllers will get
data from eSPI IP in silicon, but their drivers do not need to be 
changed, run the same as LPC
mode.

You can image bellowing work path:

  KCS    Mailbox  Snoop (Port 80)  UART ....
    ^        ^                 ^                          ^
    |         |                |                           |
    |         |                |                           |
     \        |                /                          /
              { LPC IP }            <-------------------- { eSPI IP to 
decode the mmio address }

And in our first generation eSPI x86 server, we  just use the eSPI new 
function to decode the VW to
boot the PCH (eSPI master). Other functions such as GPIO SMBus, we 
didn't use it. So for making
things clean, we just keep the basic code, which is verified and tested 
well.
>         Arnd
---
BR,
Haiyue

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