Re: remove the ->mapping_error method from dma_map_ops V2 - Russell King

From: Russell King - ARM Linux <linux@armlinux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>,
	linux-arch@vger.kernel.org, linux-ia64@vger.kernel.org,
	linux-parisc@vger.kernel.org, robin.murphy@arm.com,
	the arch/x86 maintainers <x86@kernel.org>,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
	iommu@lists.linux-foundation.org, linux-alpha@vger.kernel.org,
	xen-devel@lists.xenproject.org,
	David Woodhouse <dwmw2@infradead.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: remove the ->mapping_error method from dma_map_ops V2
Date: Wed, 28 Nov 2018 17:45:45 +0000	[thread overview]
Message-ID: <20181128174545.GJ30658@n2100.armlinux.org.uk> (raw)
In-Reply-To: <CAHk-=wg5+rXK8dpjkSmK7tuz29VfxVWNEwmTVYB409mmyLkeew@mail.gmail.com>

On Wed, Nov 28, 2018 at 08:47:05AM -0800, Linus Torvalds wrote:
> On Tue, Nov 27, 2018 at 11:41 PM Christoph Hellwig <hch@lst.de> wrote:
> >
> > On Fri, Nov 23, 2018 at 07:55:11AM +0100, Christoph Hellwig wrote:
> > > Well, I can tweak the last patch to return -EINVAL from dma_mapping_error
> > > instead of the old 1 is as bool true.  The callers should all be fine,
> > > although I'd have to audit them.  Still wouldn't help with being able to
> > > return different errors.
> >
> > Any opinions?  I'd really like to make some forward progress on this
> > series.
> 
> So I do think that yes, dma_mapping_error() should return an error
> code, not 0/1.
> 
> But I was really hoping that the individual drivers themselves could
> return error codes. Right now the patch-series has code like this:
> 
>       ret = needs_bounce(dev, dma_addr, size);
>       if (ret < 0)
> -         return ARM_MAPPING_ERROR;
> +         return DMA_MAPPING_ERROR;
> 
> which while it all makes sense in the context of this patch-series, I
> *really* think it would have been so much nicer to return the error
> code 'ret' instead (which in this case is -E2BIG).
> 
> I don't think this is a huge deal, but ERR_PTR() has been hugely
> successful elsewhere. And I'm not hugely convinced about all these
> "any address can be valid" arguments. How the hell do you generate a
> random dma address in the last page that isn't even page-aligned?

kmalloc() a 64-byte buffer, dma_map_single() that buffer.  If you
have RAM that maps to a _bus_ address in the top page of 4GB of a
32-bit bus address, then you lose.  Simples.

Subsystems like I2C, SPI, USB etc all deal with small kmalloc'd
buffers and their drivers make use of DMA.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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