Re: [PATCH 1/2][next] scsi: qla2xxx: Replace one-element array with DECLARE_FLEX_ARRAY() helper

[Christophe JAILLET]

Le 19/11/2022 à 09:56, Gustavo A. R. Silva a écrit :
> On Sat, Nov 19, 2022 at 09:44:02AM +0100, Christophe JAILLET wrote:
> > Le 19/11/2022 à 00:47, Gustavo A. R. Silva a écrit :
> > > One-element arrays as fake flex arrays are deprecated and we are moving
> > > towards adopting C99 flexible-array members, instead. So, replace
> > > one-element array declaration in struct ct_sns_gpnft_rsp, which is
> > > ultimately being used inside a union:
> > >
> > > drivers/scsi/qla2xxx/qla_def.h:
> > > 3240 struct ct_sns_gpnft_pkt {
> > > 3241         union {
> > > 3242                 struct ct_sns_req req;
> > > 3243                 struct ct_sns_gpnft_rsp rsp;
> > > 3244         } p;
> > > 3245 };
> > >
> > > Important to mention is that doing a build before/after this patch results
> > > in no binary differences.
> >
> > Hi,
> >
> > even with the:
> >
> > >    		rspsz = sizeof(struct ct_sns_gpnft_rsp) +
> > > -			((vha->hw->max_fibre_devices - 1) *
> > > +			(vha->hw->max_fibre_devices *
> > >    			    sizeof(struct ct_sns_gpn_ft_data));
> >
> > change ?
>
> Yep; that change compensates for the removal of the 1 in the declaration
> of entries[].
>
> The above piece of code is a common idiom to calculate the size for an
> allocation when a one-element array is involved. In the original code
> (vha->hw->max_fibre_devices - 1) compensates for the _extra_ size of one
> element of type struct ct_sns_gpn_ft_data in sizeof(struct ct_sns_gpnft_rsp).

Yes, I do agree, that the code is equivalent. I was surprised that a 
compiler was smart enough to generate the same binary code.

With gcc 11.3.0 (x86_64), I do get some differences when I do:
   make drivers/scsi/qla2xxx/qla_gs.o
   objdump -D  drivers/scsi/qla2xxx/qla_gs.o > before.asm

   patch -p1 < patch.diff

   make drivers/scsi/qla2xxx/qla_gs.o
   objdump -D  drivers/scsi/qla2xxx/qla_gs.o > after.asm

   diff -u before.asm after.asm

Mostly some slight ordering of instruction changes, but the binary are 
not the same.

CJ

> --
> Gustavo
>
> > CJ
> >
> > > This help us make progress towards globally enabling
> > > -fstrict-flex-arrays=3 [1].
> > >
> > > Link: https://github.com/KSPP/linux/issues/245
> > > Link: https://github.com/KSPP/linux/issues/193
> > > Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
> > > Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> > > ---
> > >    drivers/scsi/qla2xxx/qla_def.h | 4 ++--
> > >    drivers/scsi/qla2xxx/qla_gs.c  | 2 +-
> > >    2 files changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/scsi/qla2xxx/qla_def.h b/drivers/scsi/qla2xxx/qla_def.h
> > > index a26a373be9da..1eea977ef426 100644
> > > --- a/drivers/scsi/qla2xxx/qla_def.h
> > > +++ b/drivers/scsi/qla2xxx/qla_def.h
> > > @@ -3151,12 +3151,12 @@ struct ct_sns_gpnft_rsp {
> > >    		uint8_t vendor_unique;
> > >    	};
> > >    	/* Assume the largest number of targets for the union */
> > > -	struct ct_sns_gpn_ft_data {
> > > +	DECLARE_FLEX_ARRAY(struct ct_sns_gpn_ft_data {
> > >    		u8 control_byte;
> > >    		u8 port_id[3];
> > >    		u32 reserved;
> > >    		u8 port_name[8];
> > > -	} entries[1];
> > > +	}, entries);
> > >    };
> > >    /* CT command response */
> > > diff --git a/drivers/scsi/qla2xxx/qla_gs.c b/drivers/scsi/qla2xxx/qla_gs.c
> > > index 64ab070b8716..69d3bc795f90 100644
> > > --- a/drivers/scsi/qla2xxx/qla_gs.c
> > > +++ b/drivers/scsi/qla2xxx/qla_gs.c
> > > @@ -4073,7 +4073,7 @@ int qla24xx_async_gpnft(scsi_qla_host_t *vha, u8 fc4_type, srb_t *sp)
> > >    		sp->u.iocb_cmd.u.ctarg.req_size = GPN_FT_REQ_SIZE;
> > >    		rspsz = sizeof(struct ct_sns_gpnft_rsp) +
> > > -			((vha->hw->max_fibre_devices - 1) *
> > > +			(vha->hw->max_fibre_devices *
> > >    			    sizeof(struct ct_sns_gpn_ft_data));
> > >    		sp->u.iocb_cmd.u.ctarg.rsp = dma_alloc_coherent(&vha->hw->pdev->dev,