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

[Tomi Valkeinen]

On 10/04/2024 18:25, Ville Syrjälä wrote:
> 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 at 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 at iki.fi>
>>> Cc: Tomi Valkeinen <tomi.valkeinen at ideasonboard.com>
>>> Signed-off-by: Ville Syrjälä <ville.syrjala at 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?

If I recall right, there is (was?) no iowrite64. The bus is 32 bit 
anyway. But the two 32 bit registers written with the tilcdc_write64() 
have an annoying HW race, so we tried to find a method of writing them 
that reduces the chance of race to a minimum.

  Tomi