Re: [PATCHSET] block: fix PIO cache coherency bug

[James Bottomley]

On Fri, 2006-01-13 at 18:20 +0000, Russell King wrote:
> I think you're misunderstanding the issue.  I'll give you essentially
> my understanding of the explaination that Dave Miller gave me a number
> of years ago.  This is from memory, so Dave may wish to correct it.
> 
> 1. When a driver DMAs data into a page cache page, it is written directly
>    to RAM and is made visible to the kernel mapping via the DMA API.  As
>    a result, there will be no cache lines associated with the kernel
>    mapping at the point when the driver hands the page back to the page
>    cache.

Yes ... that's essentially what I said: DMA API makes us kernel
coherent.  However, we explicitly *don't* mandate the way architectures
do this.  It's certainly true, all the ones I know work by flushing the
kernel mapping cache lines, but I don't think you're entitled to rely on
this behaviour ... it's not inconcievable for large external cache
machines that the DMA could be done straight into the kernel cache.

>    However, in the PIO case, there is the possibility that the data read
>    from the device into the kernel mapping results in cache lines
>    associated with the page.  Moreover, if the cache is write-allocate,
>    you _will_ have cache lines.
> 
>    Therefore, you have two completely differing system states depending
>    on how the driver decided to transfer data from the device to the page
>    cache.
> 
>    As such, drivers must ensure that PIO data transfers have the same
>    system state guarantees as DMA data transfers.
> 
>    ISTR davem recommended flush_dcache_page() be used for this.

Ah ... perhaps this is the misunderstanding.  To clear the kernel lines
associated with the page you use flush_kernel_dcache_page().
flush_dcache_page() is used to make a page cache page coherent with its
user mappings.

> 2. (this is my own)  The cachetlb document specifies quite clearly what
>    is required whenever a page cache page is written to - that is
>    flush_dcache_page() is called.  The situation when a driver uses PIO
>    quote clearly violates the requirements set out in that document.
> 
> >From (2), it is quite clear that flush_dcache_page() is the correct
> function to use, otherwise we would end up with random set of state
> of pages in the page cache.  (1) merely reinforces that it's the
> correct place for the decision to be made.  In fact, it's the only
> part of the kernel which _knows_ what needs to be done.

True, but your assumption that a driver should be doing this is what I'm
saying is incorrect.  A driver's job is to deliver data coherently to
the kernel.  The kernel's job is to deliver it coherently to the user.

Perhaps we should take this to linux-arch ... the audience there is well
versed in these arcane problems?

James


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