RE: [PATCH v4 3/4] soc: aspeed: Add eSPI driver

[ChiaWei Wang <chiawei_wang@aspeedtech.com>]

Hi Jeremy,

> From: Jeremy Kerr <jk@codeconstruct.com.au>
> Sent: Thursday, September 9, 2021 9:53 AM
>  
> > Yes, there is security concern using HW mode.
> > Our designer is considering to remove the HW mode support in the next
> > generation of Aspeed SoCs.
> > So far we haven't encountered a scenario demanding HW mode.
> 
> Great to hear :) can we unconditionally set ESPI000[9] in the driver then?

Yes for the v5 revision. As HW mode is going to be eliminated in the future.
A brief explanation will also be added.

> 
> > > With than in mind, if we're disabling hardware mode - what does the
> > > direction control setting effect when we're in software mode
> > > (ESPICTRL[9] == 1)? Does it even matter?
> >
> > Yes, the direction matters even in SW mode.
> > When the direction is 'master-to-slave' and the GPIO value is updated
> > by the Host through PUT_VW, a VW interrupt is trigger to notify BMC.
> > For the 'slave-to-master' GPIO, a alert is generated to notify the
> > Host to issue GET_VW for the GPIO value updated by the BMC by ESPI09C.
> 
> OK, but the datasheet mentions that ESPICFG804 is only applicable when
> ESPI000[9] = 0, or is that not the case?

Yes, ESPICFG804 is applicable only on HW mode (ESPI000[9]=0).
When the HW mode is selected, the eSPI slave forwards GPIO update in PUT_VW packet sent by the Host to the physical GPIO based on the ESPICFG804 mapping.
This is purely done by HW. No interrupts will be generated to notify SW.

> 
> But based on what you've said: yes, it sounds like the generic gpiodev parts
> won't be useful for this.
> 
> > > Separate from this: I'm also proposing that we represent the system
> > > event VWs as gpiodevs as well.
> > >
> > > > A raw packet, primitive interface should have better flexibility
> > > > to manage MCTP packets over the OOB channel.
> > >
> > > OK, let me phrase this differently: can the OOB channel be used for
> > > anything other than SMBus messaging? Is it useful to provide an
> > > interface that isn't a standard SMBus/i2c device?
> >
> > Yes, the PCH spec. also defines two additional packet format for an
> > eSPI slave to retrieve PCH Temperature Data and RTC time.
> > It should be trivial to prepare a byte buffer in that format and send
> > it through the raw packet interface.
> 
> OK, understood.
> 
> > > If you need custom uapi definitions for this driver, that might be
> > > okay, but it's going to be more work for you (to define an interface
> > > that can be supported long-term), rather than using standard
> > > infrastructure that already exists.
> >
> > Thus I suggested that we can refer to the IPMI KCS BMC driver, which
> > supports the selection of different user interfaces, RAW or IPMI.
> 
> Yep, but the KCS "raw" register set is standardised as part of the IPMI spec too,
> which helps to define a stable user API. We don't have that in this case.
> 
> Overall though, if you want to start with the "low-level" API, then introduce
> "enhanced" functionality - like an actual SMBus driver - alongside that, then
> that sounds like an OK approach.
> 
> > If IOCTL is considered to be not user friendly or magic code
> > polluting, file-based read/write on the miscdevice node is also an
> > option.
> 
> It's not really my decision to make, but a read/write event interface would
> seem to be more consistent to me. Is there an obvious event format that would
> be common across all channels, perhaps? We'd probably also need a poll too -
> to make use of incoming events, like master-to-slave VW changes, perhaps?

A file-based read/write/poll interface is OK to me as well. The main concern is about the peripheral and the VW channels.
For the peripheral channel, it takes two miscdevice to export TX/RX interfaces for posted and non-posted packets, respectively.
And for the VW channel, the settings GPIO direction is RO and that of GPIO value is RW. And these two should be exported individually.