Re: [PATCH] drm/panfrost: Update io-pgtable API

From: Alyssa Rosenzweig <alyssa@collabora.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: tomeu.vizoso@collabora.com, dri-devel@lists.freedesktop.org,
	steven.price@arm.com, alyssa.rosenzweig@collabora.com,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] drm/panfrost: Update io-pgtable API
Date: Wed, 31 Aug 2022 08:49:16 -0400	[thread overview]
Message-ID: <Yw9YzJD7S9R+cRwp@maud> (raw)
In-Reply-To: <bbff1b5b-020c-7f61-1530-20fb0d6a1ef9@arm.com>

On Tue, Aug 23, 2022 at 11:42:33AM +0100, Robin Murphy wrote:
> On 2022-08-23 03:51, Alyssa Rosenzweig wrote:
> > > -static size_t get_pgsize(u64 addr, size_t size)
> > > +static size_t get_pgsize(u64 addr, size_t size, size_t *count)
> > >   {
> > > -	if (addr & (SZ_2M - 1) || size < SZ_2M)
> > > -		return SZ_4K;
> > > +	size_t blk_offset = -addr % SZ_2M;
> > 
> > addr is unsigned. if this is correct, it's magic.
> 
> Eh, it's just well-defined unsigned integer overflow. Take "SZ_2M - (addr %
> SZ_2M)", realise the first term can be anything that's zero modulo SZ_2M,
> including zero, then also that the operations can be done in either order to
> give the same result, and there you go.

Shrug. It still seems voodoo to me but if this is normal kernel style
I'm not going to complain, Acked-by for the patch regardless.