linux-kernel - Re: [PATCH v1 0/4] char: xillybus: Remove usage of the deprecated 'pci-dma-compat.h' API

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Date:   Sat, 28 Aug 2021 23:26:26 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Eli Billauer <eli.billauer@...il.com>
Cc:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Arnd Bergmann <arnd@...db.de>,
        gregkh <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel-janitors@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v1 0/4] char: xillybus: Remove usage of the deprecated
 'pci-dma-compat.h' API

On Sat, Aug 28, 2021 at 5:07 PM Eli Billauer <eli.billauer@...il.com> wrote:
>
> On 27/08/21 20:17, Christophe JAILLET wrote:
> > In [1], Christoph Hellwig has proposed to remove the wrappers in
> > include/linux/pci-dma-compat.h.
> >
> Xillybus' driver is an example for why this is a good idea. But has this
> been decided upon? Are we sure that there isn't a single platform where
> the DMA mapping for PCI is different from non-PCI, and that such
> platform will never be?

Yes.

> If so, is there any reference to that decision?

The documentation was updated 11 years ago to only describe the modern
linux/dma-mapping.h interfaces and mark the old bus-specific ones as
no longer recommended, see 216bf58f4092 ("Documentation: convert
PCI-DMA-mapping.txt to use the generic DMA API").

> I think the best way is to put a comment at the top of pci-dma-compat.h
> saying that the functions in that header file are deprecated and will go
> away soon. That would, more than anything else, convince people like me
> to get rid of those PCI-DMA function calls.

The only reason for keeping the old interface around any day longer would
be to identify drivers that have been unmaintained for the past decade
and ignored all the previous cleanup patches that got sent to them.

       Arnd