About the "USB Cache and busdma usage in USB" thread

[Rafal Jaworowski]

On 2009-07-25, at 05:34, M. Warner Losh wrote:

> In message: <200907232209.47729.hselasky at c2i.net>
>            Hans Petter Selasky <hselasky at c2i.net> writes:
> : On Thursday 23 July 2009 20:53:06 Marcel Moolenaar wrote:
> : > All,
> : >
> : > I went over the thread and this is what I have to say about it:
> : >
> : > Using busdma to manage/control CPU caches is wrong for the
> : > following simple reason: bus_dmamap_sync() has the side-effect
> : > of copying to and from the bounce buffer (if applicable).
> : >
> : > CPU caches should be kept coherent by using an appropriate API.
> : > We already have cpu_flush_dcache(). All we have to do is add
> : > cpu_inval_dcache() and let the MD code determine how best to
> : > do this -- even if they decide to use busdma.
> : >
> : > In general: D-cache and I-cache control/handling should not be
> : > hidden from MI code. It should not be treated as an artifact of
> : > some platform. It should not be implemented by banking on some
> : > side-effect of other function(s). We only achieve efficient
> : > cache control if MI code calls appropriate APIs so that we can
> : > precisely express what we need to achieve at that point.
> : >
> : > For example: when we write a breakpoint into the text segment
> : > of some process by using ptrace(2), the ptrace(2) code must
> : > call an appropriate API to make sure that the I-cache is made
> : > coherent with memory. This may require a previous D-cache
> : > flush! We should not kluge uiomove(9) like we did on PowerPC
> : > to deal with this. Note ARM and ia64 are still broken in this
> : > respect.
> :
> : Hi,
> :
> : I would be fine with a solution where cpufunctions are used  
> directly in USB.
> : The only problem is that if bounce pages are used, which happens  
> in the case
> : of loading kernel virtual data into DMA, then busdma sync calls  
> would still be
> : required.
>
> They are needed on i386 kernels with more than 4GB of ram...  Or ram
> located above 4GB...

Hans,
So how do you want to proceed with these cache sync issues? We need to  
fix this before 8.0.

Rafal