[U-Boot] [PATCH 1/1] arm: armv7: enable unaligned access

[U-Boot] [PATCH 1/1] arm: armv7: enable unaligned access

[Heinrich Schuchardt]

We use the command bootefi to run UEFI executables like GRUB and iPXE.
The UEFI spec requires that unaligned access is enabled if the CPU
supports it. This is true for armv7.

So we should not set bit 1 of the system control register, the alignment
bit.

Without this patch iPXE snp.efi cannot be executed on the Allwinner A20.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
---
 arch/arm/cpu/armv7/start.S | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/arm/cpu/armv7/start.S b/arch/arm/cpu/armv7/start.S
index 7e2695761e..1771741119 100644
--- a/arch/arm/cpu/armv7/start.S
+++ b/arch/arm/cpu/armv7/start.S
@@ -150,7 +150,6 @@ ENTRY(cpu_init_cp15)
 	mrc	p15, 0, r0, c1, c0, 0
 	bic	r0, r0, #0x00002000	@ clear bits 13 (--V-)
 	bic	r0, r0, #0x00000007	@ clear bits 2:0 (-CAM)
-	orr	r0, r0, #0x00000002	@ set bit 1 (--A-) Align
 	orr	r0, r0, #0x00000800	@ set bit 11 (Z---) BTB
 #ifdef CONFIG_SYS_ICACHE_OFF
 	bic	r0, r0, #0x00001000	@ clear bit 12 (I) I-cache
-- 
2.11.0

[U-Boot] [PATCH 1/1] arm: armv7: enable unaligned access

[Siarhei Siamashka]

On Thu, 29 Mar 2018 23:33:50 +0200
Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:

> We use the command bootefi to run UEFI executables like GRUB and iPXE.
> The UEFI spec requires that unaligned access is enabled if the CPU
> supports it. This is true for armv7.
> 
> So we should not set bit 1 of the system control register, the alignment
> bit.
> 
> Without this patch iPXE snp.efi cannot be executed on the Allwinner A20.
> 
> Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
> ---
>  arch/arm/cpu/armv7/start.S | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/arch/arm/cpu/armv7/start.S b/arch/arm/cpu/armv7/start.S
> index 7e2695761e..1771741119 100644
> --- a/arch/arm/cpu/armv7/start.S
> +++ b/arch/arm/cpu/armv7/start.S
> @@ -150,7 +150,6 @@ ENTRY(cpu_init_cp15)
>  	mrc	p15, 0, r0, c1, c0, 0
>  	bic	r0, r0, #0x00002000	@ clear bits 13 (--V-)
>  	bic	r0, r0, #0x00000007	@ clear bits 2:0 (-CAM)
> -	orr	r0, r0, #0x00000002	@ set bit 1 (--A-) Align
>  	orr	r0, r0, #0x00000800	@ set bit 11 (Z---) BTB
>  #ifdef CONFIG_SYS_ICACHE_OFF
>  	bic	r0, r0, #0x00001000	@ clear bit 12 (I) I-cache

Can you postpone flipping this bit until the very moment when you
are about to start your UEFI executable?

The main reason against setting this bit for the whole U-Boot
globally is that a lot of common code in U-Boot can be run on
different CPU architectures, including those which don't
support unaligned memory accesses (ARMv5, MIPS, ...). This
is a maintenance nightmare. Because the people, who test their
patches only on ARMv7 hardware, will unintentionally keep
breaking other architectures.

-- 
Best regards,
Siarhei Siamashka

[U-Boot] [PATCH 1/1] arm: armv7: enable unaligned access

[Heinrich Schuchardt]

On 03/30/2018 01:36 AM, Siarhei Siamashka wrote:
> On Thu, 29 Mar 2018 23:33:50 +0200
> Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
> 
>> We use the command bootefi to run UEFI executables like GRUB and iPXE.
>> The UEFI spec requires that unaligned access is enabled if the CPU
>> supports it. This is true for armv7.
>>
>> So we should not set bit 1 of the system control register, the alignment
>> bit.
>>
>> Without this patch iPXE snp.efi cannot be executed on the Allwinner A20.
>>
>> Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
>> ---
>>  arch/arm/cpu/armv7/start.S | 1 -
>>  1 file changed, 1 deletion(-)
>>
>> diff --git a/arch/arm/cpu/armv7/start.S b/arch/arm/cpu/armv7/start.S
>> index 7e2695761e..1771741119 100644
>> --- a/arch/arm/cpu/armv7/start.S
>> +++ b/arch/arm/cpu/armv7/start.S
>> @@ -150,7 +150,6 @@ ENTRY(cpu_init_cp15)
>>  	mrc	p15, 0, r0, c1, c0, 0
>>  	bic	r0, r0, #0x00002000	@ clear bits 13 (--V-)
>>  	bic	r0, r0, #0x00000007	@ clear bits 2:0 (-CAM)
>> -	orr	r0, r0, #0x00000002	@ set bit 1 (--A-) Align
>>  	orr	r0, r0, #0x00000800	@ set bit 11 (Z---) BTB
>>  #ifdef CONFIG_SYS_ICACHE_OFF
>>  	bic	r0, r0, #0x00001000	@ clear bit 12 (I) I-cache
> 
> Can you postpone flipping this bit until the very moment when you
> are about to start your UEFI executable?
> 
> The main reason against setting this bit for the whole U-Boot
> globally is that a lot of common code in U-Boot can be run on
> different CPU architectures, including those which don't
> support unaligned memory accesses (ARMv5, MIPS, ...). This
> is a maintenance nightmare. Because the people, who test their
> patches only on ARMv7 hardware, will unintentionally keep
> breaking other architectures.
> 

Hello Siarhei,

we could clear the bit when the bootefi command is invoked.

Best regards

Heinrich

[U-Boot] [PATCH 1/1] arm: armv7: enable unaligned access

[Heinrich Schuchardt]

On 03/30/2018 02:00 AM, Heinrich Schuchardt wrote:
> On 03/30/2018 01:36 AM, Siarhei Siamashka wrote:
>> On Thu, 29 Mar 2018 23:33:50 +0200
>> Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
>>
>>> We use the command bootefi to run UEFI executables like GRUB and iPXE.
>>> The UEFI spec requires that unaligned access is enabled if the CPU
>>> supports it. This is true for armv7.
>>>
>>> So we should not set bit 1 of the system control register, the alignment
>>> bit.
>>>
>>> Without this patch iPXE snp.efi cannot be executed on the Allwinner A20.
>>>
>>> Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
>>> ---
>>>  arch/arm/cpu/armv7/start.S | 1 -
>>>  1 file changed, 1 deletion(-)
>>>
>>> diff --git a/arch/arm/cpu/armv7/start.S b/arch/arm/cpu/armv7/start.S
>>> index 7e2695761e..1771741119 100644
>>> --- a/arch/arm/cpu/armv7/start.S
>>> +++ b/arch/arm/cpu/armv7/start.S
>>> @@ -150,7 +150,6 @@ ENTRY(cpu_init_cp15)
>>>  	mrc	p15, 0, r0, c1, c0, 0
>>>  	bic	r0, r0, #0x00002000	@ clear bits 13 (--V-)
>>>  	bic	r0, r0, #0x00000007	@ clear bits 2:0 (-CAM)
>>> -	orr	r0, r0, #0x00000002	@ set bit 1 (--A-) Align
>>>  	orr	r0, r0, #0x00000800	@ set bit 11 (Z---) BTB
>>>  #ifdef CONFIG_SYS_ICACHE_OFF
>>>  	bic	r0, r0, #0x00001000	@ clear bit 12 (I) I-cache
>>
>> Can you postpone flipping this bit until the very moment when you
>> are about to start your UEFI executable?
>>
>> The main reason against setting this bit for the whole U-Boot
>> globally is that a lot of common code in U-Boot can be run on
>> different CPU architectures, including those which don't
>> support unaligned memory accesses (ARMv5, MIPS, ...). This
>> is a maintenance nightmare. Because the people, who test their
>> patches only on ARMv7 hardware, will unintentionally keep
>> breaking other architectures.
>>
> 
> Hello Siarhei,
> 
> we could clear the bit when the bootefi command is invoked.
> 
> Best regards
> 
> Heinrich
> 
> 

Hello Albert,

in his mail
https://lists.denx.de/pipermail/u-boot/2018-April/324719.html
Alexander Graf wrote that in his view using this patch as is the
preferable way to ensure that unaligned access is available to EFI binaries.

What is your view?

Best regards

Heinrich