[patch 2/5] Staging: vme: add VME userspace driver

Emilio G. Cota cota at braap.org
Tue Aug 11 09:40:25 UTC 2009

Martyn Welch wrote:
> suggest if you want something more complex that allows the user to just  
> pick a location/size and not worry about windows at all

That's exactly the whole point. I think each bridge should manage
its resources; putting this on the upper layer would mean the
layer should have a mechanism of 'discovering' what the bridge
can/can't do. Anyway this could be revisited later.
I'm preparing a patch for this.

>> - Most accesses are 32-bit accesses. Treating all of them
>>   as 64-bit accesses would decrease performance for most of
>>   them--which happen to be 32-bit.
> I'm not - I'm storing them as 64-bit values, which they are, in the  
> structures used in *software*. These are then split *when* a write to  
> the hardware registers is required. Similarly, when the registers are  
> occasionally read they are combined and stored as a 64-bit value. This  
> simplifies all *software* checking and manipulation. By storing these as  
> 2 32-bit values every driver that uses the VME core will need to convert  
> pci addresses, vme addresses and counts to 2 32-bit values. That is 
> madness.

I agree with you on that's painful for doing 64-bit accesses.
However I'm still not convinced on the performance side (I mean
software), since most of the time the upper 32bits will be empty.
Will have a look though.


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