linux-kernel - Re: [git pull] drm fixes for 5.19-rc7

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Date:   Sat, 16 Jul 2022 14:35:54 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nathan Chancellor <nathan@...nel.org>,
        Matthew Auld <matthew.auld@...el.com>,
        Nirmoy Das <nirmoy.das@...el.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>
Cc:     Dave Airlie <airlied@...il.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [git pull] drm fixes for 5.19-rc7

On Fri, Jul 15, 2022 at 2:09 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On Fri, Jul 15, 2022 at 01:36:17PM +1000, Dave Airlie wrote:
> > Matthew Auld (1):
> >       drm/i915/ttm: fix sg_table construction
>
> This patch breaks i386_defconfig with both GCC and clang:
>
>   ld: drivers/gpu/drm/i915/i915_scatterlist.o: in function `i915_rsgt_from_mm_node':
>   i915_scatterlist.c:(.text+0x1a7): undefined reference to `__udivdi3'

Yeah, we definitely don't want arbitrary 64x64 divides in the kernel,
and the fact that we don't include libgcc.a once again caught this
horrid issue.

The offending code is

        if (sg_alloc_table(st, DIV_ROUND_UP(node->size, segment_pages),
                           GFP_KERNEL)) {

and I have to say that all of those games with "u64 page_alignment"
that commit aff1e0b09b54 ("drm/i915/ttm: fix sg_table construction")
introduces are absolutely disgusting.

And they are just *pointlessly* disgusting.

Why is that "page_alignment" a "u64"?

And why is it a "size", instead of being a "number of bits"?

The code literally does things like

        const u64 max_segment = round_down(UINT_MAX, page_alignment);

which means that

 (a) page_alignment must be a power-of-two for this to work
(round_down() only works in powers of two)

 (b) the result obviously must fit in an "unsigned int", since it's
rounding down a UINT_MAX!

So (a) makes it doubtful that "page_alignment" should have been a
value (as opposed to mask), and (b) then questions why was that made
an "u64" value when it cannot have a u64 range?

And if max_segments cannot have a 64-bit range, then segment_pages here:

        u64 segment_pages = max_segment >> PAGE_SHIFT;

sure cannot.

Fixing those then uncovers other things:

                len = min(block_size, max_segment - sg->length);

now complains about mixing types ("max_segment - sg->length" being
u32), because 'block_size' is 64, bit, and that does seem to make some
amount of sense:

        block_size = node->size << PAGE_SHIFT;

with the 'node->size' being from drm_mm_node, and that size is a
'u64'. That I *could* see being more than 32 bits on a 64-bit
architecture. Ok.

But then that means that 'len' cannot be a 64-bit value either, and it
should probably have been

                u32 len;
    ..
                len = min_t(u64, block_size, max_segment - sg->length);

and that would just have been a lot nicer on 32-bit x86, avoiding a
lot of pointlessly 64-bit things.

That said, even those type simplifications do not fix the fundamental
issue. That "DIV_ROUND_UP()" still ends up being a 64-bit divide,
although now it's at least a "64-by-32" bit divide.

Which needs to be handled by "do_div()" rather than the generic
DIV_ROUND_UP() helper, because sadly, at least gcc still generates a
full __udivdi3() even for the 64-by-32 divides.

Can Intel GPU people please take a look?

             Linus