Re: [PATCH v3 4/9] pc-bios: s390x: Get rid of magic offsets into the lowcore

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 27.05.20 09:49, Janosch Frank wrote:
> If we have a lowcore struct that has members for offsets that we want
> to touch, why not use it?
> 
> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> ---
>  pc-bios/s390-ccw/cio.h  | 17 +++++++++++------
>  pc-bios/s390-ccw/main.c |  8 +++-----
>  2 files changed, 14 insertions(+), 11 deletions(-)
> 
> diff --git a/pc-bios/s390-ccw/cio.h b/pc-bios/s390-ccw/cio.h
> index aaa432dedd..1e5d4e92e1 100644
> --- a/pc-bios/s390-ccw/cio.h
> +++ b/pc-bios/s390-ccw/cio.h
> @@ -122,12 +122,17 @@ typedef struct schib {
>  } __attribute__ ((packed, aligned(4))) Schib;
>  
>  typedef struct subchannel_id {
> -        __u32 cssid:8;
> -        __u32:4;
> -        __u32 m:1;
> -        __u32 ssid:2;
> -        __u32 one:1;
> -        __u32 sch_no:16;
> +    union {
> +        struct {
> +            __u16 cssid:8;
> +            __u16 reserved:4;
> +            __u16 m:1;
> +            __u16 ssid:2;
> +            __u16 one:1;
> +        };
> +        __u16 sch_id;
> +    };
> +    __u16 sch_no;
>  } __attribute__ ((packed, aligned(4))) SubChannelId;
>  >  struct chsc_header {
> diff --git a/pc-bios/s390-ccw/main.c b/pc-bios/s390-ccw/main.c
> index 4e65b411e1..8b912454c9 100644
> --- a/pc-bios/s390-ccw/main.c
> +++ b/pc-bios/s390-ccw/main.c
> @@ -36,11 +36,9 @@ LowCore *lowcore; /* Yes, this *is* a pointer to address 0 */
>   */
>  void write_subsystem_identification(void)
>  {
> -    SubChannelId *schid = (SubChannelId *) 184;
> -    uint32_t *zeroes = (uint32_t *) 188;
> -
> -    *schid = blk_schid;
> -    *zeroes = 0;
> +    lowcore->subchannel_id = blk_schid.sch_id;
> +    lowcore->subchannel_nr = blk_schid.sch_no;
> +    lowcore->io_int_parm = 0;
>  }
>  
>  void write_iplb_location(void)

As an alternative, why not change the definition of Lowcore and replace both 16bit
fields lowcore->subchannel_id and lowcore->subchannel_nr with SubChannelID.
You can then simply assign lowcore->schid= blk_schid;

?