linux-kernel - Re: [PATCH 00/10] Make PCI's devres API more consistent

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Date: Wed, 17 Jan 2024 10:59:29 +0100
From: Philipp Stanner <pstanner@...hat.com>
To: andy.shevchenko@...il.com
Cc: Jonathan Corbet <corbet@....net>, Hans de Goede <hdegoede@...hat.com>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
 <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
 <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, Bjorn Helgaas
 <bhelgaas@...gle.com>, Sam Ravnborg <sam@...nborg.org>, dakr@...hat.com,
 linux-doc@...r.kernel.org,  linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org,  linux-pci@...r.kernel.org
Subject: Re: [PATCH 00/10] Make PCI's devres API more consistent

On Tue, 2024-01-16 at 23:17 +0200, andy.shevchenko@...il.com wrote:
> Mon, Jan 15, 2024 at 03:46:11PM +0100, Philipp Stanner kirjoitti:
> > ¡Hola!
> 
> i? Vim user? :-)

The Dark Side of the Force is the path to many abilities, that some
consider to be... unnatural
https://www.neo-layout.org/

> 
> > PCI's devres API suffers several weaknesses:
> > 
> > 1. There are functions prefixed with pcim_. Those are always
> > managed
> >    counterparts to never-managed functions prefixed with pci_ – or
> > so one
> >    would like to think. There are some apparently unmanaged
> > functions
> >    (all region-request / release functions, and pci_intx()) which
> >    suddenly become managed once the user has initialized the device
> > with
> >    pcim_enable_device() instead of pci_enable_device(). This
> > "sometimes
> >    yes, sometimes no" nature of those functions is confusing and
> >    therefore bug-provoking. In fact, it has already caused a bug in
> > DRM.
> >    The last patch in this series fixes that bug.
> > 2. iomappings: Instead of giving each mapping its own callback, the
> >    existing API uses a statically allocated struct tracking one
> > mapping
> >    per bar. This is not extensible. Especially, you can't create
> >    _ranged_ managed mappings that way, which many drivers want.
> > 3. Managed request functions only exist as "plural versions" with a
> >    bit-mask as a parameter. That's quite over-engineered
> > considering
> >    that each user only ever mapps one, maybe two bars.
> > 
> > This series:
> > - add a set of new "singular" devres functions that use devres the
> > way
> >   its intended, with one callback per resource.
> > - deprecates the existing iomap-table mechanism.
> > - deprecates the hybrid nature of pci_ functions.
> > - preserves backwards compatibility so that drivers using the
> > existing
> >   API won't notice any changes.
> > - adds documentation, especially some warning users about the
> >   complicated nature of PCI's devres.
> 
> Instead of adding pcim_intx(), please provide proper one for
> pci_alloc_irq_vectors(). Ideally it would be nice to deprecate
> old IRQ management functions in PCI core and delete them in the
> future.
> 

In order to deprecate the intermingling with half-managed hyprid devres
in pci.c, you need to have pci_intx() be backwards compatible. Unless
you can remove it at once.
And the least broken way to do that I thought would be pcim_intx(),
because that's consistent with how I make pci_request_region() & Co.
call into their managed counterparts.

There are 25 users of pci_intx().
We'd have to look how many of them call pcim_enable_device() and how
easy they would be to port to... pci_alloc_irq_vectors() you say? I
haven't used that before. Would have to look into it and see how we
could do that.


P.