linux-kernel - Re: [PATCH 42/44] powerpc/cell: use the dma

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Date:   Sun, 18 Jun 2017 19:54:02 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Christoph Hellwig <hch@....de>, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        xen-devel@...ts.xenproject.org, linux-c6x-dev@...ux-c6x.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-mips@...ux-mips.org, openrisc@...ts.librecores.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, dmaengine@...r.kernel.org,
        linux-tegra@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-samsung-soc@...r.kernel.org,
        iommu@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 42/44] powerpc/cell: use the dma_supported method for
 ops switching

On Sun, 2017-06-18 at 00:13 -0700, Christoph Hellwig wrote:
> On Sun, Jun 18, 2017 at 06:50:27AM +1000, Benjamin Herrenschmidt wrote:
> > What is your rationale here ? (I have missed patch 0 it seems).
> 
> Less code duplication, more modular dma_map_ops insteance.
> 
> > dma_supported() was supposed to be pretty much a "const" function
> > simply informing whether a given setup is possible. Having it perform
> > an actual switch of ops seems to be pushing it...
> 
> dma_supported() is already gone from the public DMA API as it doesn't
> make sense to be called separately from set_dma_mask.  It will be
> entirely gone in the next series after this one.

Ah ok, in that case it makes much more sense, we can rename it then.

> > What if a driver wants to test various dma masks and then pick one ?
> > 
> > Where does the API documents that if a driver calls dma_supported() it
> > then *must* set the corresponding mask and use that ?
> 
> Where is the API document for _any_ of the dma routines? (A: work in
> progress by me, but I need to clean up the mess of arch hooks before
> it can make any sense)

Heh fair enough.

> > I don't like a function that is a "boolean query" like this one to have
> > such a major side effect.
> > 
> > > From an API standpoint, dma_set_mask() is when the mask is established,
> > 
> > and thus when the ops switch should happen.
> 
> And that's exactly what happens at the driver API level.  It just turns
> out the dma_capable method is they way better place to actually
> implement it, as the ->set_dma_mask method requires lots of code
> duplication while not offering any actual benefit over ->dma_capable.
> And because of that it's gone after this series.
> 
> In theory we could rename ->dma_capable now, but it would require a
> _lot_ of churn.  Give me another merge window or two and we should
> be down to be about 2 handful of dma_map_ops instance, at which point
> we could do all this gratious renaming a lot more easily :)

Sure, I get it now, as long as it's not publicly exposed to drivers
we are fine.

Cheers,
Ben.