linux-kernel - Re: [PATCH v2 1/1] powerpc/iommu: Enable remaining IOMMU Pagesizes present in LoPAR

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Message-ID: <20210412222143.GO26583@gate.crashing.org>
Date:   Mon, 12 Apr 2021 17:21:43 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Alexey Kardashevskiy <aik@...abs.ru>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Leonardo Bras <leobras.c@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, brking@...ux.vnet.ibm.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] powerpc/iommu: Enable remaining IOMMU Pagesizes present in LoPAR

On Fri, Apr 09, 2021 at 02:36:16PM +1000, Alexey Kardashevskiy wrote:
> On 08/04/2021 19:04, Michael Ellerman wrote:
> >>>>+#define QUERY_DDW_PGSIZE_4K	0x01
> >>>>+#define QUERY_DDW_PGSIZE_64K	0x02
> >>>>+#define QUERY_DDW_PGSIZE_16M	0x04
> >>>>+#define QUERY_DDW_PGSIZE_32M	0x08
> >>>>+#define QUERY_DDW_PGSIZE_64M	0x10
> >>>>+#define QUERY_DDW_PGSIZE_128M	0x20
> >>>>+#define QUERY_DDW_PGSIZE_256M	0x40
> >>>>+#define QUERY_DDW_PGSIZE_16G	0x80
> >>>
> >>>I'm not sure the #defines really gain us much vs just putting the
> >>>literal values in the array below?
> >>
> >>Then someone says "uuuuu magic values" :) I do not mind either way. 
> >>Thanks,
> >
> >Yeah that's true. But #defining them doesn't make them less magic, if
> >you only use them in one place :)
> 
> Defining them with "QUERY_DDW" in the names kinda tells where they are 
> from. Can also grep QEMU using these to see how the other side handles 
> it. Dunno.

And *not* defining anything reduces the mental load a lot.  You can add
a comment at the single spot you use them, explaining what this is, in a
much better way!

Comments are *good*.


Segher