Date:   Mon, 29 Nov 2021 10:01:12 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Marc Zygnier <maz@kernel.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Kevin Tian <kevin.tian@intel.com>,
        Megha Dey <megha.dey@intel.com>,
        Ashok Raj <ashok.raj@intel.com>, linux-pci@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-s390@vger.kernel.org, Heiko Carstens <hca@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Jon Mason <jdmason@kudzu.us>,
        Dave Jiang <dave.jiang@intel.com>,
        Allen Hubbe <allenbh@gmail.com>, linux-ntb@googlegroups.com
Subject: Re: [patch 04/32] genirq/msi: Provide a set of advanced MSI
 accessors and iterators
Message-ID: <20211129140112.GX4670@nvidia.com>
References: <877dcsf5l5.ffs@tglx>
 <87o863e2j0.ffs@tglx>
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87o863e2j0.ffs@tglx>
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?lZomaS9M4PAj2iOczNch1wJPqcPJcysC2OPM2YqHOr76fKbgXoivR5pHNTnX?=
 =?us-ascii?Q?mAlC/KcRRiu9gd5toh3wcyScjhIYNcWyFJHRKv6idyaq3oSRGWDYV64s+8eT?=
 =?us-ascii?Q?TIo8YkZthVLVd9Ghb0ax5FLscQMZgjvkbGjQdAOgTprmOpqKfB+LgkrdcOTi?=
 =?us-ascii?Q?VOH3p7rWptvLAB58HRTHohROfRZXt2Wfum1/rFxWGsctaThRI7vEQ1eoooJ1?=
 =?us-ascii?Q?WI6uMSPx4WjFprCvNfgctzR7nVkvCVxDvSLZlQRqLjP7Y2lnExj5fC1MG6yT?=
 =?us-ascii?Q?6gwynyvTdB9sV6Z9QRFP3zQGll42XOzabTM8x3gk+GAIvzJL6uO8I0kOoM1h?=
 =?us-ascii?Q?RcpxT737/FDXpcLPCmiN8eYG5U3mkDPG5UQoEMTPAFi1KekrUMDKaKJdamyl?=
 =?us-ascii?Q?Jg5UjBVQj4PBfm8epCRSpXwV+0soZLZ20QpFceuNNzEXJtF7KTraEMgWTpN8?=
 =?us-ascii?Q?WyCPiNnFGyopohKvjalUjE0rwuAbaJh403SV0QSt8PCM/zPUyaitPxBBCH4H?=
 =?us-ascii?Q?m6R1c7qDMpPKBKgA3zMTa5mun2fwCuFRlqCoava2D/VG4Ne56f+xUdEdqNSj?=
 =?us-ascii?Q?o+vXbecvTS56AIClaVG4O5hAP+gFEehEDCo8YMAC6E0HX4KeIr/xc7XCgqzN?=
 =?us-ascii?Q?UYBvmXBRnS4n2vXDhEWkRstfEPztmu/c/NbXeha2jrG6iNJPT/bJPFexqa97?=
 =?us-ascii?Q?0tOf6Xm0boMBZ2SRv99t+2YGgEWFbAv649ecv77GBzE32EhWKkeq53wkW6PH?=
 =?us-ascii?Q?dQm5UCwH+fmyK49Uym8HcR1g4bx2lsU0gcOhNh2o5ZO4FK4d1z1YYtzNMuY9?=
 =?us-ascii?Q?K3Estd2wAKNCoYA4Cva/zlmjqZI6J8DWbevxh0HBlOXp4SrfrYhCtai5PJ+q?=
 =?us-ascii?Q?78Y4bG7soMK+9ztP1YfrGSILYDXUKGNc+ap+fchpSjEST4XCr8pxY1nyt9Zi?=
 =?us-ascii?Q?C3Jf4PQttITfjMLhORfQMo9YbolG+3B9AwBPXRawqJ++L+jEYWnVQPsXqQG6?=
 =?us-ascii?Q?wFa4GKF7l7AVaQJ602z82AVa833Qo/2lEc2BjkB1SzYW7CwP7ySpnsg8+KQV?=
 =?us-ascii?Q?X/nAC4UCxmhOv+rXvcEhl2Vu3tI6mOUxZKYa9ymZ014Tqurw4oKVlZIYuZ5P?=
 =?us-ascii?Q?stu1ZkwsLSgcCzlntNoRbYWjzjGk5ETsRfJxHT+uZqvmTz03D5dLITDCxuFy?=
 =?us-ascii?Q?BypheOe6zAbMSapA5dGW6uuSwiSF7jMyrO3qxy9368dAxyFwbrzXl++pocoQ?=
 =?us-ascii?Q?S8U0XkMPjZxEJQJMeAqxy8fpXZGttm0Ff1LtXHCaCCd5GxEX4NuPVkK3Dk4P?=
 =?us-ascii?Q?vrabxTKt+8V9zU8XqtSVs2yEYGgYOx4/SyzkFYmdfDgI1L0SP6L9urvuw/HY?=
 =?us-ascii?Q?f6YfkFyxF3LH0gXO0e8kDQJcXSEV3JQAEv7g3GJ2PjpgKGP2o/LuPEYvtuZe?=
 =?us-ascii?Q?ieEiqYUf3WkrFTWuEqHPN0SEpIuNAztODv4bz9Dj/dHVYW4MQ8hLX0DfoFDH?=
 =?us-ascii?Q?Tgs568zQ9qKwrotc7YhznStzYO/s4+R5ncKD1awCJfInsTTF652ZldWtdzmh?=
 =?us-ascii?Q?UDGwBTsEoTr02EySFfs=3D?=
X-OriginatorOrg: Nvidia.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 7d361b82-cf23-4eec-84a9-08d9b340b461
X-MS-Exchange-CrossTenant-AuthSource: BL0PR12MB5506.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 29 Nov 2021 14:01:13.7116
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: 4CpNfmPv578wse+S75EO4zvDcoD0csQQyZPM+4/QZ55QPHGq5tblPB3vDBfWq66h
X-MS-Exchange-Transport-CrossTenantHeadersStamped: BL0PR12MB5553
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Nov 29, 2021 at 10:26:11AM +0100, Thomas Gleixner wrote:
> Jason,
> 
> On Sun, Nov 28 2021 at 20:22, Thomas Gleixner wrote:
> > On Sat, Nov 27 2021 at 21:00, Jason Gunthorpe wrote:
> >> On Sat, Nov 27, 2021 at 02:22:33AM +0100, Thomas Gleixner wrote:
> >> I understand why that isn't desirable at this patch where the storage
> >> would have to be a list_head pointer, but still, seems like an odd
> >> place to end up at the end of the series.
> >>
> >> eg add index here unused and then the last patch uses it instead of
> >> __iter_idx.
> >
> > TBH, didn't think about doing just that. OTH, given the experience of
> > looking at the creative mess people create, this was probably also a
> > vain attempt to make it harder in the future.
> >
> >> Also, I don't understand why filter was stored in the dev and not
> >> passed into msi_next_desc() in the macro here?
> >
> > No real reason. I probably just stored it along with the rest. Lemme try
> > that index approach.
> 
> After looking at all the call sites again, there is no real usage for
> this local index variable.
> 
> If anything needs the index of a descriptor then it's available in the
> descriptor itself. That won't change because the low level message write
> code needs the index too and the only accessible storage there is
> msi_desc.

Oh, that makes it simpler, just use the current desc->index as the
input to the xa_for_each_start() and then there should be no need of
hidden state?

> What for? The usage sites should not have to care about the storage
> details of a facility they are using.

Generally for_each things shouldn't have hidden state that prevents
them from being nested. It is just an unexpected design pattern..

Jason