linux-kernel - Re: [PATCH 06/10] pci: move pinned status bit to pci

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Date: Wed, 17 Jan 2024 10:02:04 +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 06/10] pci: move pinned status bit to pci_dev struct

On Tue, 2024-01-16 at 23:34 +0200, andy.shevchenko@...il.com wrote:
> Mon, Jan 15, 2024 at 03:46:17PM +0100, Philipp Stanner kirjoitti:
> > The bit describing whether the PCI device is currently pinned is
> > stored
> > in the PCI devres struct. To clean up and simplify the pci-devres
> > API,
> 
> "PCI devres", 'pci-devres', ...
> Shouldn't these (and across entire series) be consistent terms?
> E.g., "PCI devres API".

Yes, we should agree on a canonical term probably.
Probably "PCI devres" is good.

> 
> > it's better if this information is stored in the pci_dev struct,
> > because
> 
> pci_dev struct --> struct pci_dev
> 
> > it allows for checking that device's pinned-status directly through
> > the
> > device struct.
> > This will later permit simplifying  pcim_enable_device().
> 
> > Move the 'pinned' boolean bit to struct pci_dev.
> 
> ...
> 
> >         u8              pm_cap;         /* PM capability offset */
> >         unsigned int    enabled:1;      /* Whether this dev is
> > enabled */
> > +       unsigned int    pinned:1;       /* Whether this dev is
> > pinned */
> >         unsigned int    imm_ready:1;    /* Supports Immediate
> > Readiness */
> >         unsigned int    pme_support:5;  /* Bitmask of states from
> > which PME#
> >                                            can be generated */
> 
> First of all, I think it's better to group PM stuff, like

ACK

> 
>         u8              pm_cap;         /* PM capability offset */
>         unsigned int    pme_support:5;  /* Bitmask of states from
> which PME#
>                                            can be generated */
>         unsigned int    imm_ready:1;    /* Supports Immediate
> Readiness */
>         unsigned int    enabled:1;      /* Whether this dev is
> enabled */
>         unsigned int    pinned:1;       /* Whether this dev is pinned
> */
> 
> Second, does this layout anyhow related to the HW layout? (For
> example,
> PME bits and their location in some HW register vs. these bitfields)
> If so, but not sure, it might be good to preserve (to some extent)
> the
> order.

As far as I know, one of the greatest weaknesses of C is that neither
Ritchie nor the standard ever said anything about the fields' order.
Hence, every field-order is purely implementation-defined and in no way
portable.
So I can't imagine a bitfield will ever directly mapp to a HW-layout?

The fields order is purely aesthetic / for the human reader.


P.

>