linux-kernel - Re: [patch 2/5] Staging: vme: add VME userspace driver

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Message-ID: <20090811094025.GC3251@braap.org>
Date:	Tue, 11 Aug 2009 11:40:25 +0200
From:	"Emilio G. Cota" <cota@...ap.org>
To:	Martyn Welch <martyn.welch@...anuc.com>
Cc:	Greg K-H <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	devel@...uxdriverproject.org,
	Sebastien Dugue <sebastien.dugue@...l.net>
Subject: Re: [patch 2/5] Staging: vme: add VME userspace driver

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.

Cheers,
E.
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