linux-kernel - Re: [PATCH 3/3] drm/xe/xe2: fix 64-bit division in pte_update

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Date: Sun, 25 Feb 2024 21:47:41 -0600
From: Lucas De Marchi <lucas.demarchi@...el.com>
To: Arnd Bergmann <arnd@...nel.org>
CC: Oded Gabbay <ogabbay@...nel.org>, Thomas Hellström
	<thomas.hellstrom@...ux.intel.com>, Arnd Bergmann <arnd@...db.de>, "Maarten
 Lankhorst" <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
	<mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
	<airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, Rodrigo Vivi
	<rodrigo.vivi@...el.com>, Matt Roper <matthew.d.roper@...el.com>, "Matthew
 Brost" <matthew.brost@...el.com>, Himal Prasad Ghimiray
	<himal.prasad.ghimiray@...el.com>, Francois Dugast
	<francois.dugast@...el.com>, Dave Airlie <airlied@...hat.com>,
	<intel-xe@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] drm/xe/xe2: fix 64-bit division in pte_update_size

On Sat, Feb 24, 2024 at 01:15:01PM +0100, Arnd Bergmann wrote:
>From: Arnd Bergmann <arnd@...db.de>
>
>This function does not build on 32-bit targets when the compiler
>fails to reduce DIV_ROUND_UP() into a shift:
>
>ld.lld: error: undefined symbol: __aeabi_uldivmod
>>>> referenced by xe_migrate.c
>>>>               drivers/gpu/drm/xe/xe_migrate.o:(pte_update_size) in archive vmlinux.a
>
>There are two instances in this function. Change the first to
>use an open-coded shift with the same behavior, and the second
>one to a 32-bit calculation, which is sufficient here as the size
>is never more than 2^32 pages (16TB).
>
>Fixes: ea97a66a2218 ("drm/xe: Disable 32bits build")

same comment as in patch 2... should rather be the commit *enabling* 32b
builds?

Should this be something to be dealt with at the DIV_ROUND_UP() layer?
Why did the compiler fail to reduce it here but didn't in the other
11 uses in the xe driver?

thanks
Lucas De Marchi

>Signed-off-by: Arnd Bergmann <arnd@...db.de>
>---
> drivers/gpu/drm/xe/xe_migrate.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/drivers/gpu/drm/xe/xe_migrate.c b/drivers/gpu/drm/xe/xe_migrate.c
>index a66fdf2d2991..ee1bb938c493 100644
>--- a/drivers/gpu/drm/xe/xe_migrate.c
>+++ b/drivers/gpu/drm/xe/xe_migrate.c
>@@ -462,7 +462,7 @@ static u32 pte_update_size(struct xe_migrate *m,
> 	} else {
> 		/* Clip L0 to available size */
> 		u64 size = min(*L0, (u64)avail_pts * SZ_2M);
>-		u64 num_4k_pages = DIV_ROUND_UP(size, XE_PAGE_SIZE);
>+		u32 num_4k_pages = (size + XE_PAGE_SIZE - 1) >> XE_PTE_SHIFT;
>
> 		*L0 = size;
> 		*L0_ofs = xe_migrate_vm_addr(pt_ofs, 0);
>-- 
>2.39.2
>