[PATCH] drm/i915: use static const array for PICK macro

[PATCH] drm/i915: use static const array for PICK macro

[Arnd Bergmann]

The varargs macro trick in _PIPE3/_PHY3/_PORT3 was meant as an optimization
to shrink the i915 kernel module by around 1000 bytes. However, the
downside is a size regression with CONFIG_KASAN, as I found from stack size
warnings with gcc-7.0.1:

before:
drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_get_hw_state':
drivers/gpu/drm/i915/intel_dpll_mgr.c:1644:1: error: the frame size of 176 bytes is larger than 100 bytes [-Werror=frame-larger-than=]
drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_enable':
drivers/gpu/drm/i915/intel_dpll_mgr.c:1548:1: error: the frame size of 224 bytes is larger than 100 bytes [-Werror=frame-larger-than=]

after:
drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_get_hw_state':
drivers/gpu/drm/i915/intel_dpll_mgr.c:1644:1: error: the frame size of 1016 bytes is larger than 1000 bytes [-Werror=frame-larger-than=]
drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_enable':
drivers/gpu/drm/i915/intel_dpll_mgr.c:1548:1: error: the frame size of 1960 bytes is larger than 1000 bytes [-Werror=frame-larger-than=]

I also checked the module sizes and got with gcc-7.0.1

original:
   text	   data	    bss	    dec	    hex	filename
2380830	1155436	   4448	3540714	 3606ea	drivers/gpu/drm/i915/i915-kasan.o
1298054	 543692	   2884	1844630	 1c2596	drivers/gpu/drm/i915/i915-nokasan.o

after ce64645d86ac:
   text	   data	    bss	    dec	    hex	filename
2389515	1154476	   4448	3548439	 362517	drivers/gpu/drm/i915/i915-kasan.o
1299639	 543692	   2884	1846215	 1c2bc7	drivers/gpu/drm/i915/i915-nokasan.o

with this patch:
   text	   data	    bss	    dec	    hex	filename
2381275	1163884	   4448	3549607	 3629a7	drivers/gpu/drm/i915/i915-kasan.o
1296038	 543692	   2884	1842614	 1c1db6	drivers/gpu/drm/i915/i915-nokasan.o

Actually showing a code size growth in .text both with and without kasan,
and my version gets most of it back at the expense of larger .data when
kasan is enabled.

Fixes: ce64645d86ac ("drm/i915: use variadic macros and arrays to choose port/pipe based registers")
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80114
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/gpu/drm/i915/i915_reg.h | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 04c8f69fcc62..39b53878a188 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -48,7 +48,7 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
 	return !i915_mmio_reg_equal(reg, INVALID_MMIO_REG);
 }
 
-#define _PICK(__index, ...) (((const u32 []){ __VA_ARGS__ })[__index])
+#define _PICK(__index, ...) ({static const u32 __arr[] = { __VA_ARGS__ }; __arr[__index];})
 
 #define _PIPE(pipe, a, b) ((a) + (pipe)*((b)-(a)))
 #define _MMIO_PIPE(pipe, a, b) _MMIO(_PIPE(pipe, a, b))
@@ -2657,10 +2657,10 @@ enum skl_disp_power_wells {
 /*
  * Clock control & power management
  */
-#define _DPLL_A (dev_priv->info.display_mmio_offset + 0x6014)
-#define _DPLL_B (dev_priv->info.display_mmio_offset + 0x6018)
-#define _CHV_DPLL_C (dev_priv->info.display_mmio_offset + 0x6030)
-#define DPLL(pipe) _MMIO_PIPE3((pipe), _DPLL_A, _DPLL_B, _CHV_DPLL_C)
+#define _DPLL_A			0x6014
+#define _DPLL_B			0x6018
+#define _CHV_DPLL_C		0x6030
+#define DPLL(pipe) _MMIO(dev_priv->info.display_mmio_offset + _PIPE3((pipe), _DPLL_A, _DPLL_B, _CHV_DPLL_C))
 
 #define VGA0	_MMIO(0x6000)
 #define VGA1	_MMIO(0x6004)
@@ -2756,10 +2756,10 @@ enum skl_disp_power_wells {
 #define   SDVO_MULTIPLIER_SHIFT_HIRES		4
 #define   SDVO_MULTIPLIER_SHIFT_VGA		0
 
-#define _DPLL_A_MD (dev_priv->info.display_mmio_offset + 0x601c)
-#define _DPLL_B_MD (dev_priv->info.display_mmio_offset + 0x6020)
-#define _CHV_DPLL_C_MD (dev_priv->info.display_mmio_offset + 0x603c)
-#define DPLL_MD(pipe) _MMIO_PIPE3((pipe), _DPLL_A_MD, _DPLL_B_MD, _CHV_DPLL_C_MD)
+#define _DPLL_A_MD				0x601c
+#define _DPLL_B_MD				0x6020
+#define _CHV_DPLL_C_MD				0x603c
+#define DPLL_MD(pipe) _MMIO(dev_priv->info.display_mmio_offset + _PIPE3((pipe), _DPLL_A_MD, _DPLL_B_MD, _CHV_DPLL_C_MD))
 
 /*
  * UDI pixel divider, controlling how many pixels are stuffed into a packet.
-- 
2.9.0

Re: [PATCH] drm/i915: use static const array for PICK macro

[Jani Nikula]

On Mon, 20 Mar 2017, Arnd Bergmann <arnd@arndb.de> wrote:
> The varargs macro trick in _PIPE3/_PHY3/_PORT3 was meant as an optimization
> to shrink the i915 kernel module by around 1000 bytes.

Really, I didn't care one bit about the size shrink, I only cared about
making it easier and less error prone to increase the number of args in
a number of places. Maintainability and correctness were the goals. Just
for the record. ;)

Otherwise, this seems like an acceptable approach to me. Would be
interesting to see what happens with this, and f4c3a88e5f04 ("drm/i915:
Tighten mmio arrays for MIPI_PORT") reverted on top.

BR,
Jani.



> However, the
> downside is a size regression with CONFIG_KASAN, as I found from stack size
> warnings with gcc-7.0.1:
>
> before:
> drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_get_hw_state':
> drivers/gpu/drm/i915/intel_dpll_mgr.c:1644:1: error: the frame size of 176 bytes is larger than 100 bytes [-Werror=frame-larger-than=]
> drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_enable':
> drivers/gpu/drm/i915/intel_dpll_mgr.c:1548:1: error: the frame size of 224 bytes is larger than 100 bytes [-Werror=frame-larger-than=]
>
> after:
> drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_get_hw_state':
> drivers/gpu/drm/i915/intel_dpll_mgr.c:1644:1: error: the frame size of 1016 bytes is larger than 1000 bytes [-Werror=frame-larger-than=]
> drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_enable':
> drivers/gpu/drm/i915/intel_dpll_mgr.c:1548:1: error: the frame size of 1960 bytes is larger than 1000 bytes [-Werror=frame-larger-than=]
>
> I also checked the module sizes and got with gcc-7.0.1
>
> original:
>    text	   data	    bss	    dec	    hex	filename
> 2380830	1155436	   4448	3540714	 3606ea	drivers/gpu/drm/i915/i915-kasan.o
> 1298054	 543692	   2884	1844630	 1c2596	drivers/gpu/drm/i915/i915-nokasan.o
>
> after ce64645d86ac:
>    text	   data	    bss	    dec	    hex	filename
> 2389515	1154476	   4448	3548439	 362517	drivers/gpu/drm/i915/i915-kasan.o
> 1299639	 543692	   2884	1846215	 1c2bc7	drivers/gpu/drm/i915/i915-nokasan.o
>
> with this patch:
>    text	   data	    bss	    dec	    hex	filename
> 2381275	1163884	   4448	3549607	 3629a7	drivers/gpu/drm/i915/i915-kasan.o
> 1296038	 543692	   2884	1842614	 1c1db6	drivers/gpu/drm/i915/i915-nokasan.o
>
> Actually showing a code size growth in .text both with and without kasan,
> and my version gets most of it back at the expense of larger .data when
> kasan is enabled.
>
> Fixes: ce64645d86ac ("drm/i915: use variadic macros and arrays to choose port/pipe based registers")
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80114
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/gpu/drm/i915/i915_reg.h | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 04c8f69fcc62..39b53878a188 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -48,7 +48,7 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
>  	return !i915_mmio_reg_equal(reg, INVALID_MMIO_REG);
>  }
>  
> -#define _PICK(__index, ...) (((const u32 []){ __VA_ARGS__ })[__index])
> +#define _PICK(__index, ...) ({static const u32 __arr[] = { __VA_ARGS__ }; __arr[__index];})
>  
>  #define _PIPE(pipe, a, b) ((a) + (pipe)*((b)-(a)))
>  #define _MMIO_PIPE(pipe, a, b) _MMIO(_PIPE(pipe, a, b))
> @@ -2657,10 +2657,10 @@ enum skl_disp_power_wells {
>  /*
>   * Clock control & power management
>   */
> -#define _DPLL_A (dev_priv->info.display_mmio_offset + 0x6014)
> -#define _DPLL_B (dev_priv->info.display_mmio_offset + 0x6018)
> -#define _CHV_DPLL_C (dev_priv->info.display_mmio_offset + 0x6030)
> -#define DPLL(pipe) _MMIO_PIPE3((pipe), _DPLL_A, _DPLL_B, _CHV_DPLL_C)
> +#define _DPLL_A			0x6014
> +#define _DPLL_B			0x6018
> +#define _CHV_DPLL_C		0x6030
> +#define DPLL(pipe) _MMIO(dev_priv->info.display_mmio_offset + _PIPE3((pipe), _DPLL_A, _DPLL_B, _CHV_DPLL_C))
>  
>  #define VGA0	_MMIO(0x6000)
>  #define VGA1	_MMIO(0x6004)
> @@ -2756,10 +2756,10 @@ enum skl_disp_power_wells {
>  #define   SDVO_MULTIPLIER_SHIFT_HIRES		4
>  #define   SDVO_MULTIPLIER_SHIFT_VGA		0
>  
> -#define _DPLL_A_MD (dev_priv->info.display_mmio_offset + 0x601c)
> -#define _DPLL_B_MD (dev_priv->info.display_mmio_offset + 0x6020)
> -#define _CHV_DPLL_C_MD (dev_priv->info.display_mmio_offset + 0x603c)
> -#define DPLL_MD(pipe) _MMIO_PIPE3((pipe), _DPLL_A_MD, _DPLL_B_MD, _CHV_DPLL_C_MD)
> +#define _DPLL_A_MD				0x601c
> +#define _DPLL_B_MD				0x6020
> +#define _CHV_DPLL_C_MD				0x603c
> +#define DPLL_MD(pipe) _MMIO(dev_priv->info.display_mmio_offset + _PIPE3((pipe), _DPLL_A_MD, _DPLL_B_MD, _CHV_DPLL_C_MD))
>  
>  /*
>   * UDI pixel divider, controlling how many pixels are stuffed into a packet.

-- 
Jani Nikula, Intel Open Source Technology Center

Re: [PATCH] drm/i915: use static const array for PICK macro

[Arnd Bergmann]

On Tue, Mar 21, 2017 at 9:26 AM, Jani Nikula
<jani.nikula@linux.intel.com> wrote:
> On Mon, 20 Mar 2017, Arnd Bergmann <arnd@arndb.de> wrote:
>> The varargs macro trick in _PIPE3/_PHY3/_PORT3 was meant as an optimization
>> to shrink the i915 kernel module by around 1000 bytes.
>
> Really, I didn't care one bit about the size shrink, I only cared about
> making it easier and less error prone to increase the number of args in
> a number of places. Maintainability and correctness were the goals. Just
> for the record. ;)

Ok. My only interest here is the warning about possible stack overflow,
though the fact that KASAN considers the array code to be fragile is
an indication that it is perhaps actually dangerous: if we ever run into
a bug that causes the array index to overflow, we might in theory
have a security bug that lets users access arbitrary kernel pointers.

While the risk for that actually happening is very low, the original code
was safer in that regard. My patch on top of yours merely turns a
hypothetical arbitrary stack access into an arbitrary .data access,
and I don't even know which one would be worse.

     Arnd

Re: [Intel-gfx] [PATCH] drm/i915: use static const array for PICK macro

[Daniel Vetter]

On Tue, Mar 21, 2017 at 09:44:07AM +0100, Arnd Bergmann wrote:
> On Tue, Mar 21, 2017 at 9:26 AM, Jani Nikula
> <jani.nikula@linux.intel.com> wrote:
> > On Mon, 20 Mar 2017, Arnd Bergmann <arnd@arndb.de> wrote:
> >> The varargs macro trick in _PIPE3/_PHY3/_PORT3 was meant as an optimization
> >> to shrink the i915 kernel module by around 1000 bytes.
> >
> > Really, I didn't care one bit about the size shrink, I only cared about
> > making it easier and less error prone to increase the number of args in
> > a number of places. Maintainability and correctness were the goals. Just
> > for the record. ;)
> 
> Ok. My only interest here is the warning about possible stack overflow,
> though the fact that KASAN considers the array code to be fragile is
> an indication that it is perhaps actually dangerous: if we ever run into
> a bug that causes the array index to overflow, we might in theory
> have a security bug that lets users access arbitrary kernel pointers.
> 
> While the risk for that actually happening is very low, the original code
> was safer in that regard. My patch on top of yours merely turns a
> hypothetical arbitrary stack access into an arbitrary .data access,
> and I don't even know which one would be worse.

Even without these arrays, if userspace could control the index we feed
into these you get arbitrary mmio access. Or semi-arbitrary at least.

None of these are bugs we should ever let through, and I think with the
current code design (where the driver constructs structs that contain the
right indizes, and userspace only ever gets to point at these structs
using an idr lookup) none of these are likely to happen.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [Intel-gfx] [PATCH] drm/i915: use static const array for PICK macro

[Jani Nikula]

On Tue, 21 Mar 2017, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Tue, Mar 21, 2017 at 09:44:07AM +0100, Arnd Bergmann wrote:
>> On Tue, Mar 21, 2017 at 9:26 AM, Jani Nikula
>> <jani.nikula@linux.intel.com> wrote:
>> > On Mon, 20 Mar 2017, Arnd Bergmann <arnd@arndb.de> wrote:
>> >> The varargs macro trick in _PIPE3/_PHY3/_PORT3 was meant as an optimization
>> >> to shrink the i915 kernel module by around 1000 bytes.
>> >
>> > Really, I didn't care one bit about the size shrink, I only cared about
>> > making it easier and less error prone to increase the number of args in
>> > a number of places. Maintainability and correctness were the goals. Just
>> > for the record. ;)
>> 
>> Ok. My only interest here is the warning about possible stack overflow,
>> though the fact that KASAN considers the array code to be fragile is
>> an indication that it is perhaps actually dangerous: if we ever run into
>> a bug that causes the array index to overflow, we might in theory
>> have a security bug that lets users access arbitrary kernel pointers.
>> 
>> While the risk for that actually happening is very low, the original code
>> was safer in that regard. My patch on top of yours merely turns a
>> hypothetical arbitrary stack access into an arbitrary .data access,
>> and I don't even know which one would be worse.
>
> Even without these arrays, if userspace could control the index we feed
> into these you get arbitrary mmio access. Or semi-arbitrary at least.
>
> None of these are bugs we should ever let through, and I think with the
> current code design (where the driver constructs structs that contain the
> right indizes, and userspace only ever gets to point at these structs
> using an idr lookup) none of these are likely to happen.

That's all true, but I'm curious if explicit checks would help
kasan. Something like:

diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 04c8f69fcc62..0ab32a05b5d8 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -48,7 +48,8 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
 	return !i915_mmio_reg_equal(reg, INVALID_MMIO_REG);
 }
 
-#define _PICK(__index, ...) (((const u32 []){ __VA_ARGS__ })[__index])
+#define _PICK_NARGS(...) ARRAY_SIZE(((const u32 []){ __VA_ARGS__ }))
+#define _PICK(__index, ...) ((__index) >= 0 && (__index) < _PICK_NARGS(__VA_ARGS__) ? ((const u32 []){ __VA_ARGS__ })[__index] : 0)
 
 #define _PIPE(pipe, a, b) ((a) + (pipe)*((b)-(a)))
 #define _MMIO_PIPE(pipe, a, b) _MMIO(_PIPE(pipe, a, b))

---

Arnd, can you check that with kasan please? (I don't have gcc 7.) For me
the size diff against current git is

    text	   data	    bss	    dec	    hex	filename
-1137236	  31211	   2948	1171395	 11dfc3	drivers/gpu/drm/i915/i915.ko
+1139702	  31211	   2948	1173861	 11e965	drivers/gpu/drm/i915/i915.ko

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Technology Center

Re: [Intel-gfx] [PATCH] drm/i915: use static const array for PICK macro

[Arnd Bergmann]

On Tue, Mar 21, 2017 at 12:23 PM, Jani Nikula
<jani.nikula@linux.intel.com> wrote:
> On Tue, 21 Mar 2017, Daniel Vetter <daniel@ffwll.ch> wrote:
>> On Tue, Mar 21, 2017 at 09:44:07AM +0100, Arnd Bergmann wrote:
> Arnd, can you check that with kasan please? (I don't have gcc 7.) For me
> the size diff against current git is
>
>     text           data     bss     dec     hex filename
> -1137236          31211    2948 1171395  11dfc3 drivers/gpu/drm/i915/i915.ko
> +1139702          31211    2948 1173861  11e965 drivers/gpu/drm/i915/i915.ko

Sorry for the late reply.

I was rather sure that I had done the numbers and replied to you earlier, but
I see no evidence of that, so here it comes again, using gcc-7 and kasan:

   text   data    bss    dec    hex filename
2623339 511153  12064 3146556 30033c
obj-x86/drivers/gpu/drm/i915/i915-original.o
2634886 511153  12064 3158103 303057
obj-x86/drivers/gpu/drm/i915/i915-linux-next.o
2617989 520561  12064 3150614 301316
obj-x86/drivers/gpu/drm/i915/i915-arndpatch.o

The first one is linux-next with ce64645d86ac ("drm/i915: use variadic macros
and arrays to choose port/pipe based registers") reverted, the second one is
the current version, and the third is with my patch applied on top.

       Arnd

Re: [Intel-gfx] [PATCH] drm/i915: use static const array for PICK macro

[Arnd Bergmann]

On Tue, Mar 21, 2017 at 12:23 PM, Jani Nikula
<jani.nikula@linux.intel.com> wrote:
> On Tue, 21 Mar 2017, Daniel Vetter <daniel@ffwll.ch> wrote:
>> On Tue, Mar 21, 2017 at 09:44:07AM +0100, Arnd Bergmann wrote:
>>> On Tue, Mar 21, 2017 at 9:26 AM, Jani Nikula
>>> <jani.nikula@linux.intel.com> wrote:
>>> > On Mon, 20 Mar 2017, Arnd Bergmann <arnd@arndb.de> wrote:
>>> >> The varargs macro trick in _PIPE3/_PHY3/_PORT3 was meant as an optimization
>>> >> to shrink the i915 kernel module by around 1000 bytes.
>>> >
>>> > Really, I didn't care one bit about the size shrink, I only cared about
>>> > making it easier and less error prone to increase the number of args in
>>> > a number of places. Maintainability and correctness were the goals. Just
>>> > for the record. ;)
>>>
>>> Ok. My only interest here is the warning about possible stack overflow,
>>> though the fact that KASAN considers the array code to be fragile is
>>> an indication that it is perhaps actually dangerous: if we ever run into
>>> a bug that causes the array index to overflow, we might in theory
>>> have a security bug that lets users access arbitrary kernel pointers.
>>>
>>> While the risk for that actually happening is very low, the original code
>>> was safer in that regard. My patch on top of yours merely turns a
>>> hypothetical arbitrary stack access into an arbitrary .data access,
>>> and I don't even know which one would be worse.
>>
>> Even without these arrays, if userspace could control the index we feed
>> into these you get arbitrary mmio access. Or semi-arbitrary at least.
>>
>> None of these are bugs we should ever let through, and I think with the
>> current code design (where the driver constructs structs that contain the
>> right indizes, and userspace only ever gets to point at these structs
>> using an idr lookup) none of these are likely to happen.
>
> That's all true, but I'm curious if explicit checks would help
> kasan. Something like:
>
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 04c8f69fcc62..0ab32a05b5d8 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -48,7 +48,8 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
>         return !i915_mmio_reg_equal(reg, INVALID_MMIO_REG);
>  }
>
> -#define _PICK(__index, ...) (((const u32 []){ __VA_ARGS__ })[__index])
> +#define _PICK_NARGS(...) ARRAY_SIZE(((const u32 []){ __VA_ARGS__ }))
> +#define _PICK(__index, ...) ((__index) >= 0 && (__index) < _PICK_NARGS(__VA_ARGS__) ? ((const u32 []){ __VA_ARGS__ })[__index] : 0)
>
>  #define _PIPE(pipe, a, b) ((a) + (pipe)*((b)-(a)))
>  #define _MMIO_PIPE(pipe, a, b) _MMIO(_PIPE(pipe, a, b))
>
> ---
>
> Arnd, can you check that with kasan please? (I don't have gcc 7.) For me
> the size diff against current git is
>
>     text           data     bss     dec     hex filename
> -1137236          31211    2948 1171395  11dfc3 drivers/gpu/drm/i915/i915.ko
> +1139702          31211    2948 1173861  11e965 drivers/gpu/drm/i915/i915.ko
>

I just revisited my old patch when I ran into the stack size warning once
more, and realized I had not really answered your question earlier.

I compared your version to what is in 4.15-rc3 now, and to my version,
and confirmed that yours produces the largest code size of the three,
and doesn't address the warnings we get, but does cause additional
warnings ("comparison of constant '3' with boolean expression is always
true"), so that won't get us anywhere. Here are the numbers I get
with gcc-8:

   text    data     bss     dec     hex filename
2500045 486453    6912 2993410 2dad02 i915-kasan-4_14.ko
2488028 497909    6912 2992849 2daad1 i915-kasan-arnd.ko
2508814 486453    6912 3002179 2dcf43 i915-kasan-jani.ko
1639798   63269    4448 1707515 1a0dfb i915-nokasan-4.15.ko
1635284   63269    4448 1703001 19fc59 i915-nokasan-arnd.ko
1648331   63269    4448 1716048 1a2f50 i915-nokasan-jani.ko

I'll resend my old patch with the original description since I can't easily
reproduce it now without your original change, and the code has
changed again in the meantime, so I had to slightly adapt my
patch to still apply.

        Arnd