Re: [PATCH] OMAP: iommu flush page table entries from L1 and L2 cache

From: Gupta, Ramesh
Date: Fri Aug 12 2011 - 12:05:23 EST

On Thu, Aug 11, 2011 at 5:29 PM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> On Thu, Aug 11, 2011 at 02:28:39PM -0500, Gupta, Ramesh wrote:
>> Hi Russel,
> grr.

Sorry, typo.

>> On Thu, Apr 28, 2011 at 11:48 AM, Gupta, Ramesh <grgupta@xxxxxx> wrote:
>> > Hi Russel,
>> >
>> > On Thu, Apr 28, 2011 at 8:40 AM, Russell King - ARM Linux
>> > <linux@xxxxxxxxxxxxxxxx> wrote:
>> >> We _could_ invent a new API to deal with this, which is probably going
>> >> to be far better in the longer term for page table based iommus.  That's
>> >> going to need some thought - eg, do we need to pass a struct device
>> >> argument for the iommu cache flushing so we know whether we need to flush
>> >> or not (eg, if we have cache coherent iommus)...
>> my apologies for a late mail on this topic.
>> do you think of any other requirements for this new API?
>> Could we use the existing dmac_flush_range(), outer_flush_range()
>> for this purpose instead of a new API?
>> I see a comment in the arch/arm/include/asm/cacheflush.h
>> for  _not_ to use these APIs directly, but I am not really understand
>> the reason for that.
> When I create APIs, I create them to solve a _purpose_ to code which wants
> to do something.  They're not created to provide some facility which can
> be re-used for unrelated stuff.
> This has been proven many times to be the correct approach.  Over time,
> things change.

I agree.

> Let's say for arguments sake that you decide to use the DMA API stuff
> to achieve your goals.  Then, lets say that ARM DMA becomes fully
> cache coherent, but your IOMMU tables still need to be flushed from
> the L1 caches.
> Suddenly, dmac_flush_range() starts doing absolutely nothing.  Your
> driver breaks.  I get whinged at because a previously working driver
> stops working.  In effect, that usage _prevents_ me making the changes
> necessary to keep the core architecture support moving forward as
> things develop.  Or, alternatively I just ignore your driver, make the
> changes anyway and leave it to rot.
> So, APIs get created to provide a purpose.  Like - handling the DMA
> issues when mapping a buffer to DMA.  Like handling the DMA issues
> when unmapping a buffer from DMA.  If you start using those _because_
> they happen to clean or invalidate the cache for you, you're really
> asking for your driver to be broken at some point in the future.
> What is far better to do is to ensure that we have the right APIs in
> place for the purposes for which they are to be used.  So, if we need
> an API to flush out the IOMMU page table entries, then that's what
> we need, not some bodged lets-reuse-the-dma-flushing-functions thing.
> Inside the processors implementation, yes, it may well be the same
> thing, but that's a decision for the _processor_ support code to make,
> not the IOMMU writer.

I completely agree, thank you for explaining the need to use the correct API.

> As to what shape a new API should be - as I said above, maybe it should
> take a struct device argument, virtual base address and size.  Or maybe
> if we don't have any coherent IOMMUs then just ignore the device
> argument for the time being, and just pass the virtual base address and
> size.
> The next issue is whether it should require the virtual base address to
> be in the kernel direct mapped region.  If you're touching L2, then
> that's a yes, because we need to use virt_to_phys on it to get at the
> phys address for the L2 operations.
> So, I think: extend the cpu cache operations structure to have a method
> for dealing with IOMMUs.  Add an inline function to deal with calling
> that, and the L2 ops if that's what's required.

Once again thanks for the API requirements, I will work on this and send
the patches for review.

Ramesh Gupta G
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