Re: [Intel-gfx] [PATCH 1/4] drm/i915/gt: GEM_BUG_ON unexpected NULL at scatterlist walking

[Ramalingam C <ramalingam.c@intel.com>]

On 2022-04-21 at 18:57:59 +0530, Hellstrom, Thomas wrote:
> On Thu, 2022-04-21 at 17:08 +0530, Ramalingam C wrote:
> > While locating the start of ccs scatterlist in smem scatterlist, that
> > has
> > to be the size of lmem obj size + corresponding ccs data size. Report
> > bug
> > if scatterlist terminate before that length.
> >
> > Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
> > ---
> >  drivers/gpu/drm/i915/gt/intel_migrate.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_migrate.c
> > b/drivers/gpu/drm/i915/gt/intel_migrate.c
> > index 9d552f30b627..29d761da02c4 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_migrate.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_migrate.c
> > @@ -687,6 +687,12 @@ static void get_ccs_sg_sgt(struct sgt_dma *it,
> > u32 bytes_to_cpy)
> >                 bytes_to_cpy -= len;
> >
> 
> 
> >                 it->sg = __sg_next(it->sg);
> 
> If bytes_to_cpy == 0 here, couldn't it->sg be NULL then?
Hi,

bytes_to_cpy is the lmem size and the scatterlist is the length of
bytes_to_cpy + GET_CCS_BYTES(bytes_to_cpy). So this should not be null.

when bytes_to_cpy reduces to zero we will be having the start of the scatterlist
for ccs.

Ram.
> 
> > +
> > +               /*
> > +                * scatterlist supposed to be the size of
> > +                * bytes_to_cpy + GET_CCS_BYTES(bytes_to_copy).
> > +                */
> > +               GEM_BUG_ON(!it->sg);
> >                 it->dma = sg_dma_address(it->sg);
> >                 it->max = it->dma + sg_dma_len(it->sg);
> >         } while (bytes_to_cpy);
> 
> /Thomas
>