Re: [RFC PATCH 4/4] ARM: gic: use a private mapping for CPU target interfaces

On Wed, 7 Nov 2012, Will Deacon wrote:

On Tue, Nov 06, 2012 at 10:59:35PM +0000, Nicolas Pitre wrote:

quoted

On Tue, 6 Nov 2012, Will Deacon wrote:

quoted

quoted

 	/*
+	 * Get what the GIC says our CPU mask is.
+	 */
+	BUG_ON(cpu >= 8);
+	cpu_mask = readl_relaxed(dist_base + GIC_DIST_TARGET + 0);

Making the mask a u8 and using readb_relaxed here makes this bit of code
clearer to me (and the GIC apparently allows such an access to this
register).

Not always.  At least RTSM throws an exception if you do so.
Been there.

That would be a bug in the RTSM then. Have you reported it to support? (if
not, I can chase this one up). I'd rather we just fix the model than work
around it in Linux.

I have no problem with you chasing it down with the support people.

I don't want to wait for fixed RTSM versions to be released and the 
whole world to migrate to them though.

While the readl is maybe marginally unintuitive compared to a readb 
here, the code is always using readl everywhere else already, even using 
bit masking and shifting when a readb/writeb could have made the code 
much simpler (see gic_set_affinity() for example).  I therefore much 
prefer to stick to a proven 32-bit access than risking regression on 
some possible implementation where the 8-bit access wasn't properly 
implemented as the doc says it should and never exercised before.

In other words, I prefer erring on the safe side here.


Nicolas