linux-kernel - Re: [PATCH v9 2/6] PCI: endpoint: Add RC-to-EP doorbell support using platform MSI controller

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Message-ID: <87jzcfsiir.ffs@tglx>
Date: Wed, 04 Dec 2024 14:08:12 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Frank Li <Frank.li@....com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>, Krzysztof
 Wilczyński <kw@...ux.com>, Kishon Vijay Abraham I
 <kishon@...nel.org>,
 Bjorn Helgaas <bhelgaas@...gle.com>, Arnd Bergmann <arnd@...db.de>, Greg
 Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki"
 <rafael@...nel.org>, Anup Patel <apatel@...tanamicro.com>,
 linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
 imx@...ts.linux.dev, Niklas Cassel <cassel@...nel.org>,
 dlemoal@...nel.org, maz@...nel.org, jdmason@...zu.us
Subject: Re: [PATCH v9 2/6] PCI: endpoint: Add RC-to-EP doorbell support
 using platform MSI controller

On Tue, Dec 03 2024 at 18:24, Frank Li wrote:
> On Tue, Dec 03, 2024 at 11:15:27PM +0100, Thomas Gleixner wrote:
>> The fact that a MSI parent domain supports DOMAIN_BUS_DEVICE_MSI does
>> not guarantee that the parent is translation table based.
>>
>> As this is intended to be a generic library for all sorts of EP
>> implementations, there needs to be
>>
>>   - either a mechanism to prevent the initialization if the underlying
>>     MSI parent domain does not provide immutable messages
>
> How to know such information?

It obviously needs to be flagged somehow in the domain and the EP magic
needs to check that flag. 

>>
>>   - or support for endpoint specific msi_write_msg() implementations
>
> Even provide specific msi_write_msg(), write to address/data to shared
> memory.
>
> host driver:
> 1. read address/data from shared memory
> 2. write data to address.
>
> 1 and 2 is not atomic. So it can't avoid above raise conditon.

Correct. So you cannot support that case, which in turn requires to have
a mechanism to check for the immutable property.

Thanks,

        tglx