Re: [PATCH 3/3] drm/xe: Use VF_CAP_REG for device wmb

["Ghimiray, Himal Prasad" <himal.prasad.ghimiray@intel.com>]

On 03-07-2024 00:07, Michal Wajdeczko wrote:
> To force a write barrier on the device memory, we write to the
> SOFTWARE_FLAGS_SPR33 register, but this particular register was
> selected because it was one of the writable and unused register.
> 
> Since a write barrier should also work if we use the read-only
> register, switch to VF_CAP_REG register that is also marked as
> accessible for VFs.
> 
> While at it, add simple kernel-doc for xe_device_wmb() function.
> 
> Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
> Cc: Matt Roper <matthew.d.roper@intel.com>
> ---
>   drivers/gpu/drm/xe/xe_device.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_device.c b/drivers/gpu/drm/xe/xe_device.c
> index cfda7cb5df2c..74beddb55284 100644
> --- a/drivers/gpu/drm/xe/xe_device.c
> +++ b/drivers/gpu/drm/xe/xe_device.c
> @@ -744,13 +744,22 @@ void xe_device_shutdown(struct xe_device *xe)
>   {
>   }
>   
> +/**
> + * xe_device_wmb() - Device specific write memory barrier
> + * @xe: the &xe_device
> + *
> + * While wmb() is sufficient for a barrier if we use system memory, on discrete
> + * platforms with device memory we additionally need to issue a register write.
> + * Since it doesn't matter which register we write to, use the read-only VF_CAP
> + * register that is also marked as accessible by the VFs.
> + */

Thanks for adding the doc.

>   void xe_device_wmb(struct xe_device *xe)
>   {
>   	struct xe_gt *gt = xe_root_mmio_gt(xe);
>   
>   	wmb();
>   	if (IS_DGFX(xe))
> -		xe_mmio_write32(gt, SOFTWARE_FLAGS_SPR33, 0);

Since SOFTWARE_FLAGS_SPR33 is not used anywhere, it is better to do 
cleanup in xe_regs.h too. Can go as follow up patch.


For this patch.
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>

> +		xe_mmio_write32(gt, VF_CAP_REG, 0);
>   }
>   
>   /**