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Subject: Re: [PATCH v2 1/5] [ppc] Process dynamic relocations for kernel
From: Josh Poimboeuf <jpoimboe@linux.vnet.ibm.com>
To: Suzuki Poulose <suzuki@in.ibm.com>
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Date: Mon, 07 Nov 2011 09:13:39 -0600
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Cc: Nathan Miller <nathanm2@us.ibm.com>, Josh Poimboeuf <jpoimboe@us.ibm.com>,
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On Fri, 2011-11-04 at 14:06 +0530, Suzuki Poulose wrote:
> On 11/03/11 05:06, Josh Poimboeuf wrote:
> > On Tue, 2011-10-25 at 17:23 +0530, Suzuki K. Poulose wrote:
> > @@ -137,6 +137,9 @@ get_type:
> >   	lwz	r0, 8(r9)	/* r_addend */
> >   	add	r0, r0, r3	/* final addend */
> >   	stwx	r0, r4, r7	/* memory[r4+r7]) = (u32)r0 */
> > +	dcbst	r4,r7		/* flush dcache line to memory */
> > +	sync			/* wait for flush to complete */
> > +	icbi	r4,r7		/* invalidate icache line */
> 
> Doing it this way has two drawbacks :
> 
> 1) Placing it here in relocate would do the flushing for each and every update.

I agree.  My kernel had around 80,000 relocations, which means 80,000
d-cache line flushes (for a 32k d-cache) and 80,000 i-cache line
invalidates (for a 32k i-cache).  Which is obviously a little overkill.
Although I didn't notice a performance hit during boot.


> 2) I would like to keep this code as generic as possible for the PPC32 code.
> 
> Could we move this to the place from relocate is called and flush the d-cache and
> i-cache entirely ?

Why not put the cache flushing code at the end of relocate?  Would some
of the other PPC32 platforms not require the cache flushing?

My PPC32 knowledge is 4xx-centric, so please feel free to rewrite the
patch as needed to accommodate other PPC32 cores.


Thanks,
Josh