[PATCH] bsp/imxrt: Enforce alignment for devicetree

Wed Feb 16 15:24:52 UTC 2022

Hello Joel,

Am 16.02.22 um 16:04 schrieb Joel Sherrill:
> On Wed, Feb 16, 2022 at 3:22 AM Christian Mauderer
> <christian.mauderer at embedded-brains.de> wrote:
>>
>> A device tree binary has to be 8 byte aligned in memory. This is checked
>> since RTEMS commit 34052ef78cf8724dee73e9279b2c6bff8cfed234 "libfdt: Add
>> FDT alignment check to fdt_check_header()".
>> ---
>>   bsps/arm/imxrt/dts/imxrt1050-evkb.c   | 2 +-
>>   bsps/arm/imxrt/dts/imxrt1050-evkb.dts | 2 +-
>>   bsps/arm/imxrt/include/bsp.h          | 2 +-
>>   3 files changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/bsps/arm/imxrt/dts/imxrt1050-evkb.c b/bsps/arm/imxrt/dts/imxrt1050-evkb.c
>> index 301e790c6b..ecae160a8e 100644
>> --- a/bsps/arm/imxrt/dts/imxrt1050-evkb.c
>> +++ b/bsps/arm/imxrt/dts/imxrt1050-evkb.c
>> @@ -6,7 +6,7 @@
>>
>>   #include <sys/types.h>
>>
>> -const unsigned char imxrt_dtb[] = {
>> +const unsigned char imxrt_dtb[] __attribute__(( __aligned__(8) )) = {
>>     0xd0, 0x0d, 0xfe, 0xed, 0x00, 0x00, 0x3f, 0xf6, 0x00, 0x00, 0x00, 0x38,
>>     0x00, 0x00, 0x3b, 0xf8, 0x00, 0x00, 0x00, 0x28, 0x00, 0x00, 0x00, 0x11,
>>     0x00, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03, 0xbe,
>> diff --git a/bsps/arm/imxrt/dts/imxrt1050-evkb.dts b/bsps/arm/imxrt/dts/imxrt1050-evkb.dts
>> index 9310371428..dae7e8939f 100644
>> --- a/bsps/arm/imxrt/dts/imxrt1050-evkb.dts
>> +++ b/bsps/arm/imxrt/dts/imxrt1050-evkb.dts
>> @@ -31,7 +31,7 @@
>>    *     export BSP_DIR="${RTEMS_SRC_DIR}/bsps/arm/imxrt/"
>>    *     arm-rtems6-cpp -P -x assembler-with-cpp -I "${BSP_DIR}/include/" -include "${BSP_DIR}/dts/imxrt1050-evkb.dts" /dev/null | \
>>    *         dtc -O dtb -o "${BSP_DIR}/dts/imxrt1050-evkb.dtb" -b 0 -p 64
>> - *     rtems-bin2c -C -N imxrt_dtb "${BSP_DIR}/dts/imxrt1050-evkb.dtb" "${BSP_DIR}/dts/imxrt1050-evkb.c"
>> + *     rtems-bin2c -A 8 -C -N imxrt_dtb "${BSP_DIR}/dts/imxrt1050-evkb.dtb" "${BSP_DIR}/dts/imxrt1050-evkb.c"
>>    */
>>
>>   /dts-v1/;
>> diff --git a/bsps/arm/imxrt/include/bsp.h b/bsps/arm/imxrt/include/bsp.h
>> index d7e37628e5..9f3b913a17 100644
>> --- a/bsps/arm/imxrt/include/bsp.h
>> +++ b/bsps/arm/imxrt/include/bsp.h
>> @@ -57,7 +57,7 @@ extern "C" {
>>   #define BSP_FEATURE_IRQ_EXTENSION
>>
>>   #define BSP_FDT_IS_SUPPORTED
>> -extern const unsigned char imxrt_dtb[];
>> +extern const unsigned char imxrt_dtb[] __attribute__(( __aligned__(8) ));
> 
> Why not use RTEMS_ALIGNED?

Because I generated the imxrt1050-evkb.c using the new option in the 
rtems-bin2c and I wanted to use the same name for the prototypes in the 
header. If I would have used RTEMS_ALIGNED in the code generated by 
rtems-bin2c, it would have been necessary to add the header for 
RTEMS_ALIGNED in the generated code. I wanted to avoid that so 
rtems-bin2c generated code doesn't get new dependencies.

> 
> And is it assumed that if the user wants to provide a different
> device tree, they will follow the same naming? Otherwise, why
> are symbols in the public facing bsp.h.
> 

The i.MXRT doesn't have a bootloader that could provide the device tree. 
Therefore at the moment I just linked the device tree into the binary. 
Using a fixed name was the simplest method for allowing a user to 
replace the device tree blob. The bsp_fdt_get just retuns the pointer to 
that blob:

https://git.rtems.org/rtems/tree/bsps/arm/imxrt/start/bspstart.c?id=d24da94f620af3b8446a8305056bf690339bbeac#n74

The commands that I documented generate exactly that name:

https://docs.rtems.org/branches/master/user/bsps/bsps-arm.html#fdt

By the way: I will have to update the docuentation too as soon as this 
patch is pushed. Thanks for the hint.

The alternative would have been to move the bsp_fdt_get into a separate 
C file and skip the symbols in the bsp.h header. In that case, a user 
would have to add his own device tree blob and overwrite the bsp_fdt_get 
so that it returns his own blob. That seemed like an unnecessary extra step.

Best regards

Christian

>>   extern const size_t imxrt_dtb_size;
>>
>>   void *imx_get_reg_of_node(const void *fdt, int node);
>> --
>> 2.31.1
>>
>> _______________________________________________
>> devel mailing list
>> devel at rtems.org
>> http://lists.rtems.org/mailman/listinfo/devel

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