On Wed, Oct 23, 2019 at 9:32 AM Russell King - ARM Linux admin
<li...@armlinux.org.uk> wrote:
>
> On Wed, Oct 23, 2019 at 08:52:33AM -0500, Rob Herring wrote:
> > > I think this should have been done the other way around and default to
> > > coherent since most traditional OF platforms are coherent, and you
> > > can't just require those DTs to change.
> >
> > You can blame me. This was really only intended for cases where
> > coherency is configurable on a per peripheral basis and can't be
> > determined in other ways.
> >
> > The simple solution here is simply to use the compatible string for
> > the device to determine coherent or not.
>
> It really isn't that simple.
This doesn't work?:
if (IS_ENABLED(CONFIG_PPC) || of_dma_is_coherent(dev->of_node))
value |= ESDHC_DMA_SNOOP;
else
value &= ~ESDHC_DMA_SNOOP;
While I said use the compatibles, using the kconfig symbol is easier
than sorting out which compatibles are PPC SoCs. Though if that's
already done elsewhere in the driver, you could set a flag and use
that here. I'd be surprised if this was the only difference between
ARM and PPC SoCs for this block.
> There are two aspects to coherency, both of which must match:
>
> 1) The configuration of the device
> 2) The configuration of the kernel's DMA API
>
> (1) is controlled by the driver, which can make the decision any way
> it pleases.
>
> (2) on ARM64 is controlled depending on whether or not "dma-coherent"
> is specified in the device tree, since ARM64 can have a mixture of
> DMA coherent and non-coherent devices.
>
> A mismatch between (1) and (2) results in data corruption, potentially
> eating your filesystem. So, it's very important that the two match.
>
> These didn't match for the LX2160A, but, due to the way CMA was working,
> we sort of got away with it, but it was very dangerous as far as data
> safety went.
>
> Then, a change to CMA happened which moved where it was located, which
> caused a regression. Reverting the CMA changes didn't seem to be an
> option, so another solution had to be found.
>
> I started a discussion on how best to solve this:
>
> https://archive.armlinux.org.uk/lurker/thread/20190919.041320.1e53541f.en.html
>
> and the solution that the discussion came out with was the one that has
> been merged - which we now know caused a regression on PPC.
>
> Using compatible strings doesn't solve the issue: there is no way to
> tell the DMA API from the driver that the device is coherent. The
> only way to do that is via the "dma-coherent" property in DT on ARM64.
>
> To say that this is a mess is an under-statement, but we seem to have
> ended up here because of a series of piece-meal changes that don't seem
> to have been thought through enough.
>
> So, what's the right way to solve this, and ensure that the DMA API and
> device match as far as their coherency expectations go? Revert all the
> changes for sdhci-of-esdhc and CMA back to 5.0 or 5.1 state?
The other option is similar to earlier in the thread and just add to
of_dma_is_coherent():
/* Powerpc is normally cache coherent DMA */
if (IS_ENABLED(CONFIG_PPC) && !IS_ENABLED(CONFIG_NOT_COHERENT_CACHE))
return true;
We could do the all the weak arch hooks, but that seems like overkill
to me at this point.
Rob
