Re: [PATCH 12/21] drm/tilcdc: Allow build without __iowmb()

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Apr 10, 2024 at 12:06:29PM +0300, Tomi Valkeinen wrote:
> On 08/04/2024 20:04, Ville Syrjala wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > __iowmb() isn't available on most architectures. Make
> > its use optional so that the driver can be built on
> > other architectures with COMPILE_TEST=y.
> > 
> > Cc: Jyri Sarha <jyri.sarha@iki.fi>
> > Cc: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >   drivers/gpu/drm/tilcdc/tilcdc_regs.h | 2 ++
> >   1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/tilcdc/tilcdc_regs.h b/drivers/gpu/drm/tilcdc/tilcdc_regs.h
> > index f90e2dc3457c..44e4ada30fba 100644
> > --- a/drivers/gpu/drm/tilcdc/tilcdc_regs.h
> > +++ b/drivers/gpu/drm/tilcdc/tilcdc_regs.h
> > @@ -125,7 +125,9 @@ static inline void tilcdc_write64(struct drm_device *dev, u32 reg, u64 data)
> >   #if defined(iowrite64) && !defined(iowrite64_is_nonatomic)
> >   	iowrite64(data, addr);
> >   #else
> > +#ifdef __iowmb
> >   	__iowmb();
> > +#endif
> >   	/* This compiles to strd (=64-bit write) on ARM7 */
> >   	*(volatile u64 __force *)addr = __cpu_to_le64(data);
> >   #endif
> 
> As the memory barrier is an important part there, would it be better to 
> ifdef based on COMPILE_TEST, to make it clear why it's being done?

I can do that if you prefer.

I suppose the real question is why iowrite64() doesn't work
if a hand rolled version does work?

-- 
Ville Syrjälä
Intel